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.
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.
--> Please see the rest on the blog : http://blogs.technet.com/b/isablog/archive/2012/08/28/sent-items-delayed-when-publishing-outlook-anywhere-through-tmg.aspx