If we had the time, the Lumisoft C# source is probably the one to play with (with a VB.NET conversion being the first step :-) ) as I would like to have what visendo provides (a SMTP extender that works in with Microsoft's SMTP service), but with a catch-all, which visendo does not provide for free.
There is http://www.hmailserver.com which seems to be going back to open source according to the last tweet from the developer last July 7! <http://www.hmailserver.com/>But I will try http://www.mailenable.com <http://www.mailenable.com/>because it is Australian. -- Regards, Mark Hurd, B.Sc.(Ma.)(Hons.)
