I have the exactly same problem. But my mail server tries to send a mail every minut…. The problem is solved by refusing mails being relayed from 127.0.0.1
So now I get a log entry every minut that looks like this:
Oct 27 11:27:11 server postfix/qmgr[19906]: warning: connect to transport smtp: Connection refused
So now no spam is send from my server.
But this is not actually a real solution, because now I can’t use webmail to send mail (witch is bad). At first I was thinking if it was a php-script sending once per minut, but even if I try to shut down the webserver it still tries to send, witch (I think) rule out the webserver as the source of these spam mails.
So where to go next?