Timeout notification when sending email

I use Outlook 2010 for emails. When I send an email with a large attachment but less than the 20MB allowed by Outlook I get a message from Norton a few minutes after sending

"421 4.4.2 mdfmta009.tch.inty.net Error: timeout exceeded"

This is followed by details of the sender, recipient and subject.

The latest failure was a 15 MB attachment which was an encrypted log file to Blackberry which I need to send because it contains details of a problem.

Why does this happen and can I stop it?

Any help would be welcome.