Sent Items delayed when publishing Outlook Anywhere through TMG

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Problem

When publishing Exchange 2010 “Outlook Anywhere” via TMG 2010, you may find that some of your external Outlook users may intermittently experience issues sending email. They may report, when sending a new email, that the email may get “stuck” in the Outbox folder. The users may find that the email will be sent after a random number of minutes…or not at all.  Forcing a Send and Receive does not help. However, they may find that if they close and restart the Outlook client, the items are then sent.

The difficulty in troubleshooting this problem is that none of the endpoints in question will log any relative error messages. Neither the Outlook client, TMG nor the Exchange CAS server log any events or errors that appear relative to the issue.

Explanation

This turns out to be a timing issue which can result in ‘orphaned’ TCP connections. The Outlook client has a default RPC timeout of 12 minutes. The server to client default RPC timeout is 15 minutes.

In a publishing scenario that allows access from external clients, it’s not unusual to have a number of different network devices between the Outlook client and the internal Exchange CAS servers.  If the TCP connection timeout of one or more of these devices is sufficiently low enough, the TCP connection may be dropped by the device, causing the RPC connections between the Outlook client and the Exchange CAS server to drop. In our scenario, the device we’re interested in is TMG.

A TMG SP2 server has a default TCP keepalive value of 5 minutes. Therefore, it’s possible that TMG may drop the RPC connection from an ‘idle’ Outlook Anywhere client.

More information

The registry value that controls the Exchanges RPC Proxy connection timeout is:

HKLM\Software\Policies\Microsoft\Windows NT\Rpc\MinimumConnectionTimeout

The TMG servers’ registry value that controls TCP/IP keepalive time is:

HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Tcpip\Parameters\KeepAliveTime

NOTE: The value of MinimumConnectionTimeout is specified in seconds and the value of KeepAliveTime is specified in milliseconds.

Resolution

Decrease the Exchange CAS servers’ RPC Proxy timeout to be less than the TMG servers’ TCP keepalive time. As the default TCP keepalive value on TMG is 5 minutes, you can configure the CAS servers’ RPC Proxy timeout to 3 minutes (180 seconds) as follows:

HKLM\Software\Policies\Microsoft\Windows NT\Rpc\MinimumConnectionTimeout DWORD 0x000000b4 (180)

NOTE: The MinimumConnectionTimeout registry value does not exist by default. You’ll need to create it if it doesn’t exist in this location. Also note that adding and/or editing this registry value will require a reboot of the Exchange CAS server.

Don’t forget to check other devices on the network and make sure they do not have TCP timeout settings that might be lower than your newly configured RPC Proxy MinimumConnectionTimeout values.

Author

Richard Barker

Sr. Security Support Escalation Engineer

Microsoft CSS Forefront Edge Team