need independant smtp auth server

[Eric Wood]

The idea is to not mess with an existing SMTP mta server running on port 
25 along with all of its delicate configurations.  Instead I'd like to 
run another SMTP daemon on a non-standard port that simply serves as a 
simple SMTP AUTH server for my remote users.  It would be like a smtp 
proxy for authentication purposes only to relay.

Is there anything out there that's fairly light-weight other than a full 
blown mta?
thanks,
-Eric Wood