I've seen this behavior on a couple of our workstations and the problem recurs occasionally.
A user with Windows XP SP2 and Outlook 2003 will accumulate a number of messages in their Outbox. There is apparently nothing wrong with the outgoing messages except Outlook simply fails to successfully send them. No error message is given. Upon closing and re-opening Outlook the messages "magically" go out. Does anyone have any idea what could be causing this?
These Outlook installs use both a POP3/SMTP server running Courier on Linux and a Microsoft Exchange server for collecting and sending mail. The Linux server is the default account.
On the times when you cannot send, are the emails particularly large?
1. Register an important send/receive library file. Go to Start > Run and type regsvr32 inetcomm.dll
2. Check if antivirus email check is slowing sending and receiving. Disable it. Check if other programs running in the background is interfering. Disable one at a time the Firewall, antispyware, antispam, and all Norton programs.
3. In the account settings, Advanced settings, change the server timeout time from 1 min to 5 min
4. Check if outgoing mail server authentication is required. Select that option in account settings and use same settings as incoming server
7. See this troubleshooter for not sending
http://www.slipstick.com/problems/nosend.htm