Exchange 2007 & Apple Mac Mail

Hello. 

This one doesn't apply to Entourage but maybe somebody can help me.

I want to configure an Apple Mac Mail (via POP3/SMTP) in my organization. 

It's the only Mac and I'm having some problems.

Actually, I already did set up POP3 and it works great, but SMTP only
works 
for email addresses in our Domain.  Outside the domain, the server keeps 
telling me "5.7.1.  Unable to relay".

I can send mails to the outside world from Outlook Express, but I want it 
for the Apple Mac Mail also.  In Outlook Express I have to check "My
server 
requires authentication."  But I also need to check "Log on using Secure 
Password Authentication".  If I don't check that one, Outlook Express also

fails with the "Unable to relay" error message from Exchange. 
Unfortunately, 
I can't find an equivalent checkbox in Mac Mail to make it work.

I appreciate your suggestions.  Thanks a lot.