I myself use the exact same configuration (OS X 10.5.7 with Apple Mail 3.6,) and authenticate via AUTH SMTP against SpamFilter.

Can you please ensure you Mac users are using the "Password" option under the "Authentication" type for their SMTP Server configuration (found under the "Advanced" tab in their Outgoing Mails Server options?

If the users are still having issues, you could try to enable the “Debug” monitor in SpamFilter to capture the SMTP conversations with the remote server and see if we can have a better understanding on what is going on with a specific IP.
 
To do so, please go to the "Settings - Debug View" tab in SpamFilter, enter the IP address of the user (you can enter a partial IP by leaving blank the last octet, for ex aaa.bbb.ccc. ) in the "IP to monitor" box, and check the "Enable Debug Monitor" box. When you receive another email from any IP in that network, "stuff" regarding the connection should appear in that tab showing what kind of SMTP traffic is being exchanged. If this happens, could you please forward it to us (along with the portion of the SpamFilter’s activity logfile for that time?