Mac Mail can’t send email from our mail server, but thunderbird/pine/mutt can!

Hi, strange thing. I have set up a postfix server on our Redhat Linux system. Everything seems fine. Now, when I connect using Mac Mail, I can't send, but I can receive. There is nothing in the console.log on the Mac. But on the same Mac, thunderbird works just fine with both receiving and sending. I am wondering if anybody can give some hints as to where should I start looking. Thanks a lot. CY