ETRN on an SMTP Connector - Custom Connection Time problem

I am having a devil of a problem getting my Exchange 
Server to get the mails from my ISP.  I have had to 
create an SMTP connector and in there I have set the 
ISP's SmartHost IP address and also the ETRN for my site.

I have set it so that the ETRN is issued whenever a mail 
is sent out and also for a Custom Interval (15 mins).

Whenever I send out a mail from one of the Outlook 
clients then the ETRN is obviously issued as mail comes 
down right away.    If I send mail to myself from outside 
(say, from my Hotmail account) and then leave the room 
for an hour or more then the ETRN is obviously not being 
sent even though the ISDN is still up and running.

The Exchange 2000 server is in mixed mode on a mixed mode 
Windows 2000 machine.  Sharing this machine is my ISDN 
card and the ISA server.

This is the only snag which is affecting me as mail seems 
to be coming in and out alright (only if I send out a 
mail though).   Other than to write a small VB app which 
sends out a mail message to my hotmail account every 
fifteen minutes is there anything else which I could do?

Other related posts: