Outlook 2003 - duplicating email with large attachments?

I've noticed that Outlook 2003 duplicates outgoing mail when attachments are over about 3MB. This also cause the offending email to be "locked" in the Outbox. I've noticed this problem with more than one SMTP server.

I've looked on google but have been unable to find a definitive answer.

Any ideas please?

Are you sending straight to an ISPs SMTP or through one internally?

A few isps do have a size limit on their SMTP, one we still look after has a 10mb attachment limit, i know you mentioned 3mb, but how big are these attachments you are sending?

I had a similar issue with it duplicating incomming mail and my virus scanner couldn't scan them, found out that it was the PST file that had corrupted, created a new one and worked correctly after that