You can set the polling interval so that polling occurs more frequently than every 15 minutes:

Add a Schedule (DWORD) entry to
 

"HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\SmallBusinessServer\Network\POP3Connector"

To determine the actual polling interval: Divide the POP3 Scheduling interval (the value you have configured in the POP3-Connector GUI's Scheduling tab) by the value you set for the ScheduleAccelerator reg entry.  For example, if you specify a 15-minute interval in the POP3-Connector GUI, and you set the ScheduleAccelerator value to 3, the POP3 Connector will poll every 5 minutes (15 divided by 3). If you set it to 5, the POP3 Connector will poll every 3 minutes.

This is officially unsupported, but I've been using it for more than a year on four SBS2003 systems and have had no difficulties