PROPOSED STANDARD

Errata Exist

Internet Engineering Task Force (IETF) K. Moore Request for Comments: 8314 Windrock, Inc. Updates: 1939, 2595, 3501, 5068, 6186, 6409 C. Newman Category: Standards Track Oracle ISSN: 2070-1721 January 2018 Cleartext Considered Obsolete: Use of Transport Layer Security (TLS) for Email Submission and Access Abstract This specification outlines current recommendations for the use of Transport Layer Security (TLS) to provide confidentiality of email traffic between a Mail User Agent (MUA) and a Mail Submission Server or Mail Access Server. This document updates RFCs 1939, 2595, 3501, 5068, 6186, and 6409. Status of This Memo This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc8314. Copyright Notice Copyright (c) 2018 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Moore & Newman Standards Track [Page 1]

RFC 8314 Use of TLS for Email Submission/Access January 2018 1 . Introduction RFC3501], the Post Office Protocol (POP) [RFC1939], and/or Simple Mail Transfer Protocol (SMTP) Submission [RFC6409] usually has Transport Layer Security (TLS) [RFC5246] support but often does not use it in a way that maximizes end-user confidentiality. This specification describes current recommendations for the use of TLS in interactions between Mail User Agents (MUAs) and Mail Access Servers, and also between MUAs and Mail Submission Servers. In brief, this memo now recommends that: o TLS version 1.2 or greater be used for all traffic between MUAs and Mail Submission Servers, and also between MUAs and Mail Access Servers. o MUAs and Mail Service Providers (MSPs) (a) discourage the use of cleartext protocols for mail access and mail submission and (b) deprecate the use of cleartext protocols for these purposes as soon as practicable. o Connections to Mail Submission Servers and Mail Access Servers be made using "Implicit TLS" (as defined below), in preference to connecting to the "cleartext" port and negotiating TLS using the STARTTLS command or a similar command. This memo does not address the use of TLS with SMTP for message relay (where Message Submission [RFC6409] does not apply). Improving the use of TLS with SMTP for message relay requires a different approach. One approach to address that topic is described in [RFC7672]; another is provided in [MTA-STS]. The recommendations in this memo do not replace the functionality of, and are not intended as a substitute for, end-to-end encryption of electronic mail. 1.1 . How This Document Updates Previous RFCs RFC 1939), IMAP (RFC 3501), and Submission (RFC 6409, RFC 5068) in two ways: 1. By adding Implicit TLS ports as Standards Track ports for these protocols as described in Section 3. 2. By updating TLS best practices that apply to these protocols as described in Sections 4 and 5. Moore & Newman Standards Track [Page 3]

RFC 8314 Use of TLS for Email Submission/Access January 2018 RFC 2595 by replacing Section 7 of RFC 2595 with the preference for Implicit TLS as described in Sections 1 and 3 of this document, as well as by updating TLS best practices that apply to the protocols in RFC 2595 as described in Sections 4 and 5 of this document. This document updates RFC 6186 as described herein, in Section 5.1. 2 . Conventions and Terminology Used in This Document BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. The term "Implicit TLS" refers to the automatic negotiation of TLS whenever a TCP connection is made on a particular TCP port that is used exclusively by that server for TLS connections. The term "Implicit TLS" is intended to contrast with the use of STARTTLS and similar commands in POP, IMAP, SMTP Message Submission, and other protocols, that are used by the client and the server to explicitly negotiate TLS on an established cleartext TCP connection. The term "Mail Access Server" refers to a server for POP, IMAP, and any other protocol used to access or modify received messages, or to access or modify a mail user's account configuration. The term "Mail Submission Server" refers to a server for the protocol specified in [RFC6409] (or one of its predecessors or successors) for submission of outgoing messages for delivery to recipients. The term "Mail Service Provider" (or "MSP") refers to an operator of Mail Access Servers and/or Mail Submission Servers. The term "Mail Account" refers to a user's identity with an MSP, that user's authentication credentials, any user email that is stored by the MSP, and any other per-user configuration information maintained by the MSP (for example, instructions for filtering spam). Most MUAs support the ability to access multiple Mail Accounts. For each account that an MUA accesses on its user's behalf, it must have the server names, ports, authentication credentials, and other configuration information specified by the user. This information, which is used by the MUA, is referred to as "Mail Account Configuration". Moore & Newman Standards Track [Page 4]

RFC 8314 Use of TLS for Email Submission/Access January 2018 RFC5234], including the core rules provided in Appendix B of [RFC5234] and the rules provided in [RFC5322]. 3 . Implicit TLS RFC2595], [RFC3207], and [RFC3501]. With STARTTLS, the client establishes a cleartext application session and determines whether to issue a STARTTLS command based on server capabilities and client configuration. If the client issues a STARTTLS command, a TLS handshake follows that can upgrade the connection. Although this mechanism has been deployed, an alternate mechanism where TLS is negotiated immediately at connection start on a separate port (referred to in this document as "Implicit TLS") has been deployed more successfully. To encourage more widespread use of TLS and to also encourage greater consistency regarding how TLS is used, this specification now recommends the use of Implicit TLS for POP, IMAP, SMTP Submission, and all other protocols used between an MUA and an MSP. 3.1 . Implicit TLS for POP RFC7817]. Once the TLS session is established, POP3 [RFC1939] protocol messages are exchanged as TLS application data for the remainder of the TCP connection. After the server sends an +OK greeting, the server and client MUST enter the AUTHORIZATION state, even if a client certificate was supplied during the TLS handshake. See Sections 5.5 and 4.2 for additional information on client certificate authentication. See Section 7.1 for port registration information. 3.2 . Implicit TLS for IMAP RFC7817]. Once the TLS session is established, IMAP [RFC3501] protocol messages are exchanged as TLS application data for the remainder of the TCP connection. If a client certificate was provided during the TLS handshake that the server finds acceptable, the server MAY issue a PREAUTH greeting, in which case both the Moore & Newman Standards Track [Page 5]

RFC 8314 Use of TLS for Email Submission/Access January 2018 5.5 and 4.2 for additional information on client certificate authentication. See Section 7.2 for port registration information. 3.3 . Implicit TLS for SMTP Submission RFC7817]. Once the TLS session is established, Message Submission protocol data [RFC6409] is exchanged as TLS application data for the remainder of the TCP connection. (Note: The "submissions" service name is defined in Section 7.3 of this document and follows the usual convention that the name of a service layered on top of Implicit TLS consists of the name of the service as used without TLS, with an "s" appended.) The STARTTLS mechanism on port 587 is relatively widely deployed due to the situation with port 465 (discussed in Section 7.3). This differs from IMAP and POP services where Implicit TLS is more widely deployed on servers than STARTTLS. It is desirable to migrate core protocols used by MUA software to Implicit TLS over time, for consistency as well as for the additional reasons discussed in Appendix A. However, to maximize the use of encryption for submission, it is desirable to support both mechanisms for Message Submission over TLS for a transition period of several years. As a result, clients and servers SHOULD implement both STARTTLS on port 587 and Implicit TLS on port 465 for this transition period. Note that there is no significant difference between the security properties of STARTTLS on port 587 and Implicit TLS on port 465 if the implementations are correct and if both the client and the server are configured to require successful negotiation of TLS prior to Message Submission. Note that the "submissions" port provides access to a Message Submission Agent (MSA) as defined in [RFC6409], so requirements and recommendations for MSAs in that document, including the requirement to implement SMTP AUTH [RFC4954] and the requirements of Email Submission Operations [RFC5068], also apply to the submissions port. See Sections 5.5 and 4.2 for additional information on client certificate authentication. See Section 7.3 for port registration information. Moore & Newman Standards Track [Page 6]

RFC 8314 Use of TLS for Email Submission/Access January 2018 3.4 . Implicit TLS Connection Closure for POP, IMAP, and SMTP Submission [RFC793], Section 3.5)) without waiting for a TLS response from the server. 4 . Use of TLS by Mail Access Servers and Message Submission Servers Section 3.) o For compatibility with existing MUAs and existing MUA configurations, MSPs SHOULD also, in the near term, provide instances of these services that support STARTTLS. This will permit legacy MUAs to discover new availability of TLS capability on servers and may increase the use of TLS by such MUAs. However, servers SHOULD NOT advertise STARTTLS if the use of the STARTTLS command by a client is likely to fail (for example, if the server has no server certificate configured). o MSPs SHOULD advertise their Mail Access Servers and Mail Submission Servers, using DNS SRV records according to [RFC6186]. (In addition to making correct configuration easier for MUAs, this provides a way by which MUAs can discover when an MSP begins to offer TLS-based services.) Servers supporting TLS SHOULD be advertised in preference to cleartext servers (if offered). In addition, servers using Implicit TLS SHOULD be advertised in preference to servers supporting STARTTLS (if offered). (See also Section 4.5.) o MSPs SHOULD deprecate the use of cleartext Mail Access Servers and Mail Submission Servers as soon as practicable. (See Section 4.1.) Moore & Newman Standards Track [Page 7]

RFC 8314 Use of TLS for Email Submission/Access January 2018 RFC7525] or a future BCP or Standards Track revision of that document. o As soon as practicable, MSPs currently supporting Secure Sockets Layer (SSL) 2.x, SSL 3.0, or TLS 1.0 SHOULD transition their users to TLS 1.1 or later and discontinue support for those earlier versions of SSL and TLS. o Mail Submission Servers accepting mail using TLS SHOULD include in the Received field of the outgoing message the TLS ciphersuite of the session in which the mail was received. (See Section 4.3.) o All Mail Access Servers and Mail Submission Servers implementing TLS SHOULD log TLS cipher information along with any connection or authentication logs that they maintain. Additional considerations and details appear below. 4.1 . Deprecation of Services Using Cleartext and TLS Versions Less Than 1.1 The specific means employed for deprecation of cleartext Mail Access Servers and Mail Submission Servers MAY vary from one MSP to the next in light of their user communities' needs and constraints. For example, an MSP MAY implement a gradual transition in which, over time, more and more users are forbidden to authenticate to cleartext instances of these servers, thus encouraging those users to migrate to Implicit TLS. Access to cleartext servers should eventually be either (a) disabled or (b) limited strictly for use by legacy systems that cannot be upgraded. After a user's ability to authenticate to a server using cleartext is revoked, the server denying such access MUST NOT provide any indication over a cleartext channel of whether the user's authentication credentials were valid. An attempt to authenticate as such a user using either invalid credentials or valid credentials MUST both result in the same indication of access being denied. Moore & Newman Standards Track [Page 8]

RFC 8314 Use of TLS for Email Submission/Access January 2018 4.2 . Mail Server Use of Client Certificate Authentication RFC4422]. An IMAPS server MAY issue a PREAUTH greeting instead of enabling SASL EXTERNAL. 4.3 . Recording TLS Ciphersuite in "Received" Header Field RFC3848] provides trace information that can indicate that TLS was used when transferring mail. However, TLS usage by itself is not a guarantee of confidentiality or security. The TLS ciphersuite provides additional information about the level of security made available for a connection. This section Moore & Newman Standards Track [Page 9]

RFC 8314 Use of TLS for Email Submission/Access January 2018 https://www.iana.org/assignments/tls-parameters> tls-cipher-hex = "0x" 4HEXDIG tls-dh-group-clause = "group" FWS dh-group ; not to be used except immediately after tls-cipher dh-group = ALPHA *(ALPHA / DIGIT / "_" / "-") ; as registered in the IANA "TLS Supported Groups Registry" ; <https://www.iana.org/assignments/tls-parameters> 4.4 . TLS Server Certificate Requirements RFC7817] for the recommendations and requirements necessary to achieve this. If a protocol server provides service for more than one mail domain, it MAY use a separate IP address for each domain and/or a server certificate that advertises multiple domains. This will generally be necessary unless and until it is acceptable to impose the constraint that the server and all clients support the Server Name Indication (SNI) extension to TLS [RFC6066]. Mail servers supporting the SNI need to support the post-SRV hostname to interoperate with MUAs that have not implemented [RFC6186]. For more discussion of this problem, see Section 5.1 of [RFC7817]. Moore & Newman Standards Track [Page 10]

RFC 8314 Use of TLS for Email Submission/Access January 2018 4.5 . Recommended DNS Records for Mail Protocol Servers 4.5.1 . MX Records RFC4033]. This is mentioned here only for completeness, as the handling of inbound mail is out of scope for this document. 4.5.2 . SRV Records RFC6186]. MSPs SHOULD advertise servers that support Implicit TLS in preference to servers that support cleartext and/or STARTTLS operation. 4.5.3 . DNSSEC 4.5.4 . TLSA Records 4.6 . Changes to Internet-Facing Servers Moore & Newman Standards Track [Page 11]

RFC 8314 Use of TLS for Email Submission/Access January 2018 5 . Use of TLS by Mail User Agents Section 5.1 for more information.) o MUAs SHOULD be configurable to require a minimum level of confidentiality for any particular Mail Account and refuse to exchange information via any service associated with that Mail Account if the session does not provide that minimum level of confidentiality. (See Section 5.2.) o MUAs MUST NOT treat a session as meeting a minimum level of confidentiality if the server's TLS certificate cannot be validated. (See Section 5.3.) o MUAs MAY impose other minimum confidentiality requirements in the future, e.g., in order to discourage the use of TLS versions or cryptographic algorithms in which weaknesses have been discovered. o MUAs SHOULD provide a prominent indication of the level of confidentiality associated with an account configuration that is appropriate for the user interface (for example, a "lock" icon or changed background color for a visual interface, or some sort of audible indication for an audio user interface), at appropriate times and/or locations, in order to inform the user of the confidentiality of the communications associated with that account. For example, this might be done whenever (a) the user is prompted for authentication credentials, (b) the user is composing mail that will be sent to a particular submission server, (c) a list of accounts is displayed (particularly if the user can select from that list to read mail), or (d) the user is asking to view or update any configuration data that will be stored on a remote server. If, however, an MUA provides such an indication, it MUST NOT indicate confidentiality for any connection that does not at least use TLS 1.1 with certificate verification and also meet the minimum confidentiality requirements associated with that account. o MUAs MUST implement TLS 1.2 [RFC5246] or later. Earlier TLS and SSL versions MAY also be supported, so long as the MUA requires at least TLS 1.1 [RFC4346] when accessing accounts that are configured to impose minimum confidentiality requirements. Moore & Newman Standards Track [Page 12]

RFC 8314 Use of TLS for Email Submission/Access January 2018 RFC7525] or a future BCP or Standards Track revision of that document. o MUAs that are configured to not require minimum confidentiality for one or more accounts SHOULD detect when TLS becomes available on those accounts (using [RFC6186] or other means) and offer to upgrade the account to require TLS. Additional considerations and details appear below. 5.1 . Use of SRV Records in Establishing Configuration RFC6186] by changing the preference rules and adding a new SRV service label _submissions._tcp to refer to Message Submission with Implicit TLS. User-configurable MUAs SHOULD support the use of [RFC6186] for account setup. However, when using configuration information obtained via this method, MUAs SHOULD ignore advertised services that do not satisfy minimum confidentiality requirements, unless the user has explicitly requested reduced confidentiality. This will have the effect of causing the MUA to default to ignoring advertised configurations that do not support TLS, even when those advertised configurations have a higher priority than other advertised configurations. When using configuration information per [RFC6186], MUAs SHOULD NOT automatically establish new configurations that do not require TLS for all servers, unless there are no advertised configurations using TLS. If such a configuration is chosen, prior to attempting to authenticate to the server or use the server for Message Submission, the MUA SHOULD warn the user that traffic to that server will not be encrypted and that it will therefore likely be intercepted by unauthorized parties. The specific wording is to be determined by the implementation, but it should adequately capture the sense of risk, given the widespread incidence of mass surveillance of email traffic. Similarly, an MUA MUST NOT attempt to "test" a particular Mail Account configuration by submitting the user's authentication credentials to a server, unless a TLS session meeting minimum confidentiality levels has been established with that server. If minimum confidentiality requirements have not been satisfied, the MUA must explicitly warn that the user's password may be exposed to attackers before testing the new configuration. Moore & Newman Standards Track [Page 13]

RFC 8314 Use of TLS for Email Submission/Access January 2018 Section 6 of [RFC6186].) An MUA MUST NOT consult SRV records to determine which servers to use on every connection attempt, unless those SRV records are signed by DNSSEC and have a valid signature. However, an MUA MAY consult SRV records from time to time to determine if an MSP's server configuration has changed and alert the user if it appears that this has happened. This can also serve as a means to encourage users to upgrade their configurations to require TLS if and when their MSPs support it. 5.2 . Minimum Confidentiality Level Moore & Newman Standards Track [Page 14]

RFC 8314 Use of TLS for Email Submission/Access January 2018 5.3 . Certificate Validation RFC7817] and PKIX [RFC5280]. MUAs MAY also support DNS-Based Authentication of Named Entities (DANE) [RFC6698] as a means of validating server certificates in order to meet minimum confidentiality requirements. MUAs MAY support the use of certificate pinning but MUST NOT consider a connection in which the server's authenticity relies on certificate pinning as providing the minimum level of confidentiality. (See Section 5.4.) 5.4 . Certificate Pinning Section 5.1 describes one way to do this). The certificates for these servers are verified using the rules described in [RFC7817] and PKIX [RFC5280]. In the event that the certificate does not validate due to an expired certificate, a lack of an appropriate chain of trust, or a lack of an identifier match, the MUA MAY offer to create a persistent binding between that certificate and the saved hostname for the server, for use when accessing that account's servers. This is called "certificate pinning". (Note: This use of the term "certificate pinning" means something subtly different than HTTP Public Key Pinning as described in [RFC7469]. The dual use of the same term is confusing, but unfortunately both uses are well established.) Moore & Newman Standards Track [Page 15]

RFC 8314 Use of TLS for Email Submission/Access January 2018 RFC6125]. 5.5 . Client Certificate Authentication Section 7.4.6 of [RFC5246]. The requirement that the client not send a certificate not known to be acceptable to the server is pragmatic in multiple ways: the current TLS protocol provides no way for the client to know which of the potentially multiple certificates it should use; also, when the client sends a certificate, it is potentially disclosing its identity (or its user's identity) to both the server and any party with access to the transmission medium, perhaps unnecessarily and for no useful purpose. Moore & Newman Standards Track [Page 16]

RFC 8314 Use of TLS for Email Submission/Access January 2018 RFC4422], using the appropriate authentication command (AUTH for POP3 [RFC5034], AUTH for SMTP Submission [RFC4954], or AUTHENTICATE for IMAP [RFC3501]). 6 . Considerations Related to Antivirus/Antispam Software and Services RFC2979], including direct interference with this specification, and other forms of confidentiality or security reduction. An AVAS product or service is considered compatible with this specification if all IMAP, POP, and SMTP-related software (including proxies) it includes are compliant with this specification. Note that end-to-end email encryption prevents AVAS software and services from using email content as part of a spam or virus assessment. Furthermore, although a minimum confidentiality level can prevent a man-in-the-middle from introducing spam or virus content between the MUA and Submission server, it does not prevent other forms of client or account compromise. The use of AVAS services for submitted email therefore remains necessary. 7 . IANA Considerations 7.1 . POP3S Port Registration Update RFC6335]: Service Name: pop3s Transport Protocol: TCP Assignee: IESG <iesg@ietf.org> Contact: IETF Chair <chair@ietf.org> Description: POP3 over TLS protocol Reference: RFC 8314 Port Number: 995 Moore & Newman Standards Track [Page 17]

RFC 8314 Use of TLS for Email Submission/Access January 2018 7.2 . IMAPS Port Registration Update RFC6335]: Service Name: imaps Transport Protocol: TCP Assignee: IESG <iesg@ietf.org> Contact: IETF Chair <chair@ietf.org> Description: IMAP over TLS protocol Reference: RFC 8314 Port Number: 993 No changes to existing UDP port assignments for pop3s or imaps are being requested. 7.3 . Submissions Port Registration RFC6335]: Service Name: submissions Transport Protocol: TCP Assignee: IESG <iesg@ietf.org> Contact: IETF Chair <chair@ietf.org> Description: Message Submission over TLS protocol Reference: RFC 8314 Port Number: 465 This is a one-time procedural exception to the rules in [RFC6335]. This requires explicit IESG approval and does not set a precedent. Note: Since the purpose of this alternate usage assignment is to align with widespread existing practice and there is no known usage of UDP port 465 for Message Submission over TLS, IANA has not assigned an alternate usage of UDP port 465. Historically, port 465 was briefly registered as the "smtps" port. This registration made no sense, as the SMTP transport MX infrastructure has no way to specify a port, so port 25 is always used. As a result, the registration was revoked and was subsequently reassigned to a different service. In hindsight, the "smtps" registration should have been renamed or reserved rather than revoked. Unfortunately, some widely deployed mail software interpreted "smtps" as "submissions" [RFC6409] and used that port for email submission by default when an end user requested security during account setup. If a new port is assigned for the submissions service, either (a) email software will continue with unregistered use of port 465 (leaving the port registry inaccurate relative to Moore & Newman Standards Track [Page 18]

RFC 8314 Use of TLS for Email Submission/Access January 2018 7.4 . Additional Registered Clauses for "Received" Fields RFC5321], IANA has added two additional- registered-clauses for Received fields as defined in Section 4.3 of this document: o "tls": Indicates the TLS cipher used (if applicable) o "group": Indicates the Diffie-Hellman group used with the TLS cipher (if applicable) The descriptions and syntax of these additional clauses are provided in Section 4.3 of this document. 8 . Security Considerations TLS-1.3] appears to reduce this privacy risk somewhat. Moore & Newman Standards Track [Page 19]

RFC 8314 Use of TLS for Email Submission/Access January 2018 Appendix A . Design Considerations Email-TLS] ("Recommendations for use of TLS by Electronic Mail Access Protocols"). Subsequent versions merge ideas from both documents. One author of this document was also the author of RFC 2595, which became the standard for TLS usage with POP and IMAP, and the other author was perhaps the first to propose that idea. In hindsight, both authors now believe that that approach was a mistake. At this point, the authors believe that while anything that makes it easier to deploy TLS is good, the desirable end state is that these protocols always use TLS, leaving no need for a separate port for cleartext operation except to support legacy clients while they continue to be used. The separate-port model for TLS is inherently simpler to implement, debug, and deploy. It also enables a "generic TLS load-balancer" that accepts secure client connections for arbitrary foo-over-TLS protocols and forwards them to a server that may or may not support TLS. Such load-balancers cause many problems because they violate the end-to-end principle and the server loses the ability to log security-relevant information about the client unless the protocol is designed to forward that information (as this specification does for the ciphersuite). However, they can result in TLS deployment where it would not otherwise happen, which is a sufficiently important goal that it overrides any problems. Although STARTTLS appears only slightly more complex than separate-port TLS, we again learned the lesson that complexity is the enemy of security in the form of the STARTTLS command injection vulnerability (Computer Emergency Readiness Team (CERT) vulnerability ID #555316 [CERT-555316]). Although there's nothing inherently wrong with STARTTLS, the fact that it resulted in a common implementation error (made independently by multiple implementers) suggests that it is a less secure architecture than Implicit TLS. Section 7 of RFC 2595 critiques the separate-port approach to TLS. The first bullet was a correct critique. There are proposals in the HTTP community to address that, and the use of SRV records as described in RFC 6186 resolves that critique for email. The second bullet is correct as well but is not very important because useful deployment of security layers other than TLS in email is small enough to be effectively irrelevant. (Also, it's less correct than it used to be because "export" ciphersuites are no longer supported in modern versions of TLS.) The third bullet is incorrect because it misses the desirable option of "use TLS for all subsequent connections to Moore & Newman Standards Track [Page 24]

RFC 8314 Use of TLS for Email Submission/Access January 2018 RFC 6186) would not create interoperability problems but would require all client deployments, server deployments, and software to change significantly, which is contrary to the goal of promoting the increased use of TLS. Encouraging the use of STARTTLS on port 587 would not create interoperability problems, but it is unlikely to have any impact on the current undocumented use of port 465 and makes the guidance in this document less consistent. The remaining option is to document the current state of the world and support future use of port 465 for submission, as this increases consistency and ease of deployment for TLS email submission. Moore & Newman Standards Track [Page 25]

RFC 8314 Use of TLS for Email Submission/Access January 2018 POP3-over-TLS], which was the basis of the POP3 Implicit TLS text. Thanks to Russ Housley, Alexey Melnikov, and Dan Newman for review feedback. Thanks to Paul Hoffman for interesting feedback in initial conversations about this idea. Authors' Addresses Keith Moore Windrock, Inc. PO Box 1934 Knoxville, TN 37901 United States of America Email: moore@network-heretics.com Chris Newman Oracle 440 E. Huntington Dr., Suite 400 Arcadia, CA 91006 United States of America Email: chris.newman@oracle.com Moore & Newman Standards Track [Page 26]