Incoming Emails on Mac (Via SBS 2003)

In our company we have ~15 PCs and 1 mac.
I'm the computer gal and I've never used a mac before.
I got it all set up, but now I've been told that the user isn't receiving incoming, external emails.
She can send and receive internals without issue. I asked her to see if she could mail out (to her yahoo account) but she's not in today and hasn't tried it yet.

Our server is SBS 2003 (which I didn't set up; I've only been at this company a few months)

I've tried hunting for info but I can only find 'can receive but not send' or 'can't receive any' type answers.
I've seen a few things about changing such-a-setting and I know I tried all the ones I found when I first set the damn thing up.

Does anyone have any ideas where I can begin (bearing in mind that I have no idea where the various settings are on SBS, I do backups, databases etc, I've never actually looked at a server properly before)

Thanks
Mandi

re: Incoming Emails on Mac (Via SBS 2003)

I'm not an expert on SBS, but I found a site which looks promising: Configuring Incoming and Outgoing Email for Windows Small Business Server 2003

maybe it's just an issue to set up the email servers (smtp, pop3/imap) correctly, though that depends on the client used.

regards

re: Incoming Emails on Mac (Via SBS 2003)

Thanks for the post Dormilich.
I gave up in the end and asked my boss (who set it all up to begin with) and he just sorted it. Didn't let me know what he'd done, which I assume means it was something to with the ISA server as that's the only thing he hasn't shown me yet. (And I'm not prepared to go 'play' with it... I have a slight issue with messing with things when i don't know what it's going to do)