iOS 6.1 devices are hammering Exchange servers with excessive traffic, causing performance slowdowns that led Microsoft to suggest a drastic fix for the most severe cases: throttle traffic from iOS 6.1 users or block them completely.

"When a user syncs a mailbox by using an iOS 6.1-based device, Microsoft Exchange Server 2010 Client Access server (CAS) and Mailbox (MBX) server resources are consumed, log growth becomes excessive, memory and CPU use may increase significantly, and server performance is affected," Microsoft wrote on Tuesday in a support document.

The problem also affects Exchange Online in Microsoft's Office 365 cloud service. Office 365 customers may get an error message on iOS 6.1 devices stating "Cannot Get Mail: The connection to the server failed." The Microsoft support article says both Apple and Microsoft are investigating the problem.

Microsoft suggests several fixes, starting out gently, then escalating to the complete blockage of iOS 6.1 devices. Based on the fixes suggested, the problems may be caused when iOS devices connect to Exchange calendars.

The first workaround is "do not process Calendar items such as meeting requests on iOS 6.1 devices. Also, immediately restart the iOS 6.1 device."

If that doesn't work, users are instructed to remove their Exchange accounts from their phones or tablets while the Exchange Server administrator runs a "remove device" command on the server side. After 30 minutes, users can add the Exchange accounts back onto their devices but should be advised "not to process Calendar items on the device."

If that doesn't work, the fixes get more serious. The next method is for the server administrator to create a custom throttling policy limiting the number of transactions iOS 6.1 users can make with the server. "The throttling policy will reduce the effect of the issue on server resources," Microsoft notes. "However, users who receive the error should immediately restart their devices and stop additional processing of Calendar items."

One Exchange administrator who created a throttling policy through PowerShell to solve the problem provides a guide here, but Microsoft also has a page providing instructions.

Finally, the last method Microsoft recommends is to block iOS 6.1 users. "You can block iOS 6.1 users by using the Exchange Server 2010 Allow/Block/Quarantine feature," Microsoft notes. (See this post for more detailed instructions.)

Businesses of all sizes limiting or blocking iOS devices

We don't know exactly how widespread this problem is. It's clearly not affecting everyone, but the impact seems to run the gamut from small businesses to large.

"We're using Exchange 2010 in a small software firm with about a dozen iOS users (each with multiple iOS devices)," Shourya Ray, chief administrative officer of Spin Systems in Virginia, told Ars via e-mail. "Last week our Exchange server froze (internal mail was being routed, but external mail stopped flowing)."

It turned out that the 300GB VMware virtual machine hosting the Exchange server was full. "You can imagine our surprise when that VM filled up overnight," Ray said. "If we were running Exchange in a typical hardware-based server with a 1TB drive, it would have taken us a week to realize the problem."

How did it happen, and how did the company get things working "normally" again? "The transaction log had 200,000 records and was the indication of a problem," Ray said. "Our temporary solution has been to ask iOS users to switch to manual pull rather than ActiveSync push. For heavy e-mail users, we are recommending an automatic pull every 30 minutes. So far, that seems to have kept Exchange happy with no other issues since last week. Let's hope that Apple and Microsoft put their heads to together and fix this soon."

We heard from several other people on Twitter that they have been bit by the iOS 6.1/Exchange problem. One said, "My 22,000+ employee enterprise has blocked iOS 6.1, execs all have iOS."

A support thread on Microsoft's Exchange Server site was opened January 31 to discuss the excessive logging caused by iOS 6.1. The server administrator who began the thread identified an iPad that "caused over 50GB worth of logs" on a single database.

The thread got more than a dozen replies. One Exchange administrator explained that "malformed meetings on a device cause the device to get into a sync loop which causes excessive transaction log growth on the Exchange mailbox servers." This in turn "will cause Exchange performance issues and potentially transaction log drives to run out of disk space which would then bring down Exchange."

To solve the problem, this admin simply "disabled all iOS 6.1 on our Exchange system."

iOS 6.1 was released on January 28. iOS 6.1.1 came out a couple of days ago, but for now it can only be installed on the iPhone 4S and is designed to fix cellular performance and reliability. Apple didn't mention anything about Exchange fixes when releasing this latest version. Last year, iOS 6.0.1 fixed an Exchange problem that could lead to entire meetings being canceled when even a single iOS user declined a meeting invitation.

The iOS 6.1 problem isn't the first time iOS has caused Exchange servers to perform poorly. An Apple support article from 2010 describes sync problems in iOS 4 and says, "Exchange Server administrators may notice their servers running slowly." At the time, Microsoft noted iOS 4 led to "Exchange administrators... seeing heavier than normal loads on their servers from users with iOS devices." Microsoft got in touch with Apple to fix that problem.

We've asked both Apple and Microsoft how many users are impacted by the latest problem, and when a more permanent fix is coming. We also asked Apple if it agrees with the workarounds suggested by Microsoft. Microsoft told us it has nothing else to say, as the "support article contains the latest." Apple has not responded to our request for comment as of yet.

UPDATE: Apple posted a support document of its own today, describing the problem thusly:

When you respond to an exception to a recurring calendar event with a Microsoft Exchange account on a device running iOS 6.1, the device may begin to generate excessive communication with Microsoft Exchange Server. You may notice increased network activity or reduced battery life on the iOS device. This extra network activity will be shown in the logs on Exchange Server and it may lead to the server blocking the iOS device. This can occur with iOS 6.1 and Microsoft Exchange 2010 SP1 or later, or Microsoft Exchange Online (Office365).

Apple's suggested fix is to turn the Exchange calendar off and back on again within the iPhone's settings. An operating system update to fix the problem is on the way. "Apple has identified a fix and will make it available in an upcoming software update," Apple said.