As far as I understand it's works only with Mail program that comes with MacOS X but I suppose if you really need to use smtp.mac.com then you have to modify a header of the message that you send vis your mail software. it's easy to do with ResEdit. I use Outlook under MacOS 9.1 and smtp.mac.com works fine for me because I modefied the header of the message and now it's now mail server think that I use Apple Mail software from MacOS X.
Best,
MACALEX