Outlook problem sending large attachements

I am getting a problem sending large attachements in Outlook 2000. I cannot send e-mails with attachments larger that 1 Mb. The message sits in the outbox and doesn't get sent. If you try to force it by doing a manual send/receive (or F5) the message:

"The TCP/IP connection was unexpectedly terminated by your mail server. If this problem persists, contact your LAN administrator or Internet Provider ".

Smaller messages with attachments seem to go fine. E-mail are received without problem.

The System is on a single small LAN and connection is via POP and SMTP to a server on the same LAN running MDaemon v7.1.0. No errors are evident in the mail server logs and I am fairly sure that the messages are not leaving Outlook.

Google search results suggest this problem may be related to Anti-virus scanning of e-mail. However, disabling e-mail scanning in anti-virus doesn't seem to fix the problem. THe systems are running McAfee Virusscan 4.5.1 being managed by ePO 2.5.

I also removed the "Exchange Scan" Add-in using Options in Outlook and briefly it seemed to work, but very quickly the problem reoccurred without the Exchange Scan being reselected.

Searching through the sent mailbox on a couple of machines demonstrates that some large e-mail have been sent in the past without problem. 20Mb plus!.

My next steps are to :

install another e-mail client (Mozilla THunderbird) and see how that works (to determine if this is a client side problem).

uninstall the anti-virus system from one PC to see it e-mails start to work.

Anyone have any other ideas? Any suggestion gratefully received. This problem is occuring in one of our branch offices and I want to get home tonight!

 

Solution Found

Fixed it.

Uninstalling McAfee had no effect, but installing Thunderbird was useful.

I was unable to send any e-mails with Thunderbird and kept being prompted for a password in spite of me entering the right password. Resetting the password did not fix the problem.

This behaviour led me to the conclusion that I had a permissions problem at the server. To cut a long story short, I removed MDaemon 7.1 and reinstalled 6.5. Everything is now working fine. Reinstalling 7.1 seemed to end up with a system that had inherited the problem.