If you have "Use SSL to connect", try using port 465. This is the default port for SSL SMTP, and some ISPs block SSL on port 25. Mac Mail has a setting that allows it to automatically try ports 25, 465, and 587 if one or two of them don't work (though whether that setting works is sometimes hit or miss).

If you still aren't able to send via port 465, you may want to check what port the Mac Mail systems are set to.

-SW