Outlook won't send emails with attachments

[Michael Donahoe]

I have multiple pop email accounts set up in Outlook. The problem is
that it will send emails without attachments but if I try to send an
email with an attachment I get the following error: 500 Unauthorized
relay msg rejected. What's stranger is if I send it to myself (I
assume within the same domain) it will send fine and receive fine. I
often have to send it (not receive) and then forward it from my
webmail account. If I send from Outlook it immediately kicks back
with that error and does not send it to the Internet. When something
does go out Norton antivirus scans it first and then sends it.

Any ideas, I haven't been able to find a similar problem on the net.

Thanks,

Michael Donahoe