limit auth to IP

["Martin G.H. Minkler" <minkler@xxxxxxxxxx>]

Aloha!

Is it possible to make a decision whether to offer authentication to a 
user based on the querying source IP address?

One of the customers is picking up all mail for ~70 users with an 
exchange server and plain text POP3 while using criminally simple 
passwords, so we would like to limit the logins for just those accounts 
to the IP address of that exchange server.

Of course this cannot happen on a firewall level as other customers have 
to be able to log in from any arbitrary source IP and OTOH iptables 
can't look into the packets to parse the username :-)


TIA

Martin