After migrating Exchange on-premises to Exchange online for several years now. I have lived the evolution of both Exchange and Exchange Online simultaneously, migrations and hybrid. I am pleased to say that the process of migrating to Exchange Online has improved by leaps and bounds, night and day, yin and yang…OK, by now you appreciate what I am trying to get across.

Improvements in Exchange itself for Hybrid deployments and improvements in the Hybrid Config Wizard make it easier than ever to run the hybrid configuration wizard and get the party started. There are many nuances that catch administrators by surprise, negatively impact users, and consequentially hurt the IT department’s reputation. Microsoft has a TechNet page dedicated to Exchange Online limits, but time and time again I realize that this information is not common knowledge.

Since the limits are all provided by Microsoft in a TechNet article. I will only mention limits that I encounter or cause concern during migrations.

Message Limits

Message limits are important because they have many facets. Outlook clients can send attachments of 150 MB. Outlook Web Access (OWA) users can’t attach files greater than 35 MB as an individual attachment or collectively. Another difference, the message size limit for a tenant can be 150 MB but if you are using OWA as the client, the message limit is 112 MB and there isn’t a workaround for this limitation. Who sends 150 MB files? Especially, when you can share via One Drive for Business, SharePoint or FTP is beyond me. Organizations and users sometimes have unique use cases that justify the strangest use of technology.

Receiving Limits

Receiving limits are always a hot topic. Especially when you consider any mass notifications systems such as monitoring systems. A user, group or public folder can only receive up to 3,600 messages per hour, for the average organization this is not an issue. If you happen to be a Managed Service Provider this can be a serious problem. Also, a user is also capped on how many emails can be sent in one day. That magic number is 10,000, while this seems like a high number for the average person it becomes significantly small when you look at this from a marketing point of view. Yes, there are services such as Constant Contact that can handle this for you, but some smaller business may want to keep cost of marketing down and send their own marketing emails.

Distribution Groups

Distribution Groups also have some unique and sometimes hard to understand limits. You can have 100,000 members of a distribution group – great! Where is the limit you may ask? If you are emailing 5,000 or more members then the maximum message size can’t exceed 2 MB. You can get around this limitation by creating sub distribution groups and keeping the primary distribution group under 5,000 objects.

Dynamic distribution groups can experience other issues not related to limits but worth mentioning. If the dynamic distribution group is created pre- Hybrid (most organizations) the dynamic distribution group will simply not work. Microsoft does have a fix for this. Which is straightforward and easy to implement. You must enable the list to include mail-enabled users or you can recreate the distribution list in the Exchange Online.

Journaling

Journaling is another one of those sometimes-surprising revelations to customers. Microsoft states that they support Journaling in Exchange Online and this is true but there are caveats. The journaling mailbox cannot reside in Exchange Online.

There are many other caveats that need to be thoroughly explored but are out of the scope of this post. These are essential to achieving a successful Exchange Online migration. They are as follows but not limited to; mailbox permissions, auto mapping of mailboxes, Azure AD Connect, etc.

The key takeaway should be that whilst Microsoft has vastly improved the migration experience to Exchange Online there are key nuances that need to be explored and understood prior to taking the leap.

Sources

Exchange Online Limits

Members of a dynamic distribution group in an Exchange hybrid deployment don’t receive emails messages.