SMTP mail not getting sent from office server

Small Business Server 2003

[deMorte]

There's a quite a lot of background for the problem at the end of the post. If the problem doesn't make sense, I ask you to read the background.

Our Small Business Server sends all the mail that are sent thru Exchange Server but does not send mails that are sent directly to the SMTP address (the same one the Exchange Server uses). I get the following error message from Symantec Email Proxy (we have Symantec Endpoint Protection):
"Your message to ... was unable to be sent because the connection to your mail server was interrupted."
On the Knowledge Base it suggests that the problem is with the service provider, which I doubt.

I have tried to set the Endpoint to accept outgoing connections to the SMTP address but this has no effect.

I'm guessing the problem lies with the Symantec settings, but as this kind of network administration is not really my area of expertise, the problem could lie within the Server settings as well.

Any suggestions would be most welcome.

Background

Our ISP changed their SMTP addresses spring so that the companies have their own address ("smtp.company.com") and private users have their own ("smtp.private.com") where as previously they used only "smtp.private.com". And that's when the problems started.

None of our employees could send any emails from their mail programs. I changed the exchange server to use the "smtp.company.com", but the problem didn't go away. I contacted the ISP and after a long Q&A session, we finally came to the conclusion that the Symantec Antivirus on our server was blocking the mail.

That's when I found out that we didn't have the passwords to the antivirus (it was installed by the company that set up the server). I uninstalled the SAV and installed Symantec Endpoint Protection on its place on the server and on the desktops. And now we are facing the problem above.

[bill]

Did you have any luck with this? Sounds like you need to be in contact with the people from Symantec.

[deMorte]

The problem has mutated since I posted the question.

We have a laptop that uses Windows XP Home and therefore does not login to the server. User logs on to the local profile and connects to the server when needs to access web drive. This user/computer can send mail thru direct SMTP connection, so I'm thinking Symantec isn't actually to blame.

I'm now inclining to think that the Small Business Server/Exchange Server is configured wrong.
As I said, this is not really my area of expertise and would be grateful if someone could make suggestions as to where to start looking.