Firewall/NAT

I'm using my m0n0wall was a classic gateway/router to the internet. So far, everything works fine, except, that m0n0 seems to block some outbound traffic (LAN to internet).
Example from syslog:
Dec 26 23:21:46 gatekeeper/gatekeeper ipmon[89]: 23:21:45.986915 sis1 @0:15 b 192.168.1.101,56965 -> XXX.XXX.XXX.XXX,993 PR tcp len 20 142 -AFP IN

XXX.XXX.XXX.XXX stands for the IP of the IMAP/mail server of the my ISP.
192.168.1.101 is my Mac.

My default route config is the following:
      <rule>
         <type>pass</type>
         <interface>lan</interface>
         <source>
            <network>lan</network>
         </source>
         <destination>
            <any/>
         </destination>
         <descr>Default Route</descr>
      </rule>


Installed is m0n0wall 1.235.

Thanks in advance for your help!

Regards,
Jochen