Exchange 5.x will deque messages waiting for your domain over a dial-up connection.
To deque messages over a routed connection, you can schedule a batch file:
REM Cause dialup router to connect
if not %ERRORLEVEL% EQU 0 exit
REM If IMS not started, start it.
net start MSExchangeIMC
REM use 3rd party retrieval program or
REM FINGER your.domain.com@ISP.Host.domainisp.com or
REM use the AlwaysUseETRN registry value
REM stop IMS, if you started it.
net stop MSExchangeIMC
On the Edit menu, Add Value name AlwaysUseETRN, a type REG_DWORD value, and set the data to 1.
This will cause IMS to send an ETRN command to all ESMTP hosts it connects to, if they advertise support for ETRN, and are listed in the routing table as configured for IMS.