http://silverwraith.com/vexim
I've been very happy with this to date. But it's not a drop-in solution for your existing setup, rather a new way of doing things...
Otherwise you may have luck cooking up some simple CGI scripts... Though you'd probably be best off just converting to Vexim...
There is an Exim module for Webmin, though it wasn't feature-complete last time I checked, perhaps a year ago....
-AL.