I am experiencing a couple of exchange 2010 related challenges.
One, we have at least one external mail recipient we can not send to. I've tested this using the exchange mail flow troubleshooter and do not get any errors. After sending mail to this recipient; we receive a NDR delay report a couple hours later, followed by a could not send.
"Delivery has failed to these recipients or groups:
The server has tried to deliver this message, without success, and has stopped trying. Please try sending this message again. If the problem continues, contact your helpdesk."
The second issue we are having is related to exchange performance. Occasionally when replying to email, there are times when we see a message pop-up about trying to send, with a task bar, and then it sends after a few seconds. Other users have complained of similar delays after deleting an email, or simply navigating to a new mail message in Outlook.
Exchange Server is operating on a Dell PE R710 x64 w/ Server 200 R2. We have roughly 75 users and we are running a mixed XP and 7 environment, with both Outlook 2003 and Outlook 2010.