1. See if changing the Outgoing (SMTP) port to 225 helps.
2. Also disable email scanning by your anti-virus application. It's
unnecessary, it may be causing the problem, and even Symantec says it's not
needed:
<QP>
Disabling Email Scanning does not leave you unprotected against viruses that
are distributed as email attachments. Norton AntiVirus Auto-Protect scans
incoming files as they are saved to your hard drive, including email and
email attachments. Email Scanning is just another layer on top of this. To
make sure that Auto-Protect is providing the maximum protection, keep
Auto-Protect enabled and run LiveUpdate regularly to ensure that you have
the most recent virus definitions.
</QP>
http://service1.symantec.com/SUPPORT/nav.nsf/docid/2002111812533106
Why you don't need your anti-virus to scan your email
http://thundercloud.net/infoave/tutorials/email-scanning/index.htm