I have a windows 2003 server running hmailserver, and about 120 windows XP/vista/7 users are sending email through it using SMTP. Seems in the past few days we have been infected with something 'cos now we are in spam blacklists and the hmailserver logs indicate that we are sending messages with typical spam subjects from non existing accounts.
This is a very serious problem to us, so I'm thinking of two posibilities:
1- Install some kind of snifferl in all the 120 desktop clients in order to detect (and fix) whose of them are using SMTP protocols intensively.
2- Get some kind of antispam software for the server (hmailserver on Windows 2003), one that not only filters incoming messages, but most importantly, one that may prevent our server from sending messages that seems to be spam.
I think that the best would be to do both things, but I do not know what programs to use. It would be great if you suggest some software capable of doing this (I mean, generate log files about desktop SMTP protocol use, and also prevent the server from processing outgoing spam messages). I would be even better if you can suggest something free.
Also, do you know what else can I do to solve this problem?