It is possible to make Mail.app use an SMTP server running on a port other than 25. This is especially nice if you're using SSH to tunnel SMTP connections and don't want to (or cannot) forward localhost's port 25. This option is not accessible from Mail's preferences, you will have to manually edit the preferences file (com.apple.mail.plist in ~/Library/Preferences).
Read the rest of the article for the how-to...
<dict>
<key>AccountType</key>
<string>SMTPAccount</string>
<key>Hostname</key>
<string>smtp.mydomain.com</string>
<key>ShouldUseAuthentication</key>
<string>NO</string>
</dict>
<key>PortNumber</key>
<string>2342</string>
Mac OS X Hints
http://hints.macworld.com/article.php?story=20020708094743450