Was banned for spam. How to find a pest?

Question

Mail server

Good time to all.
I am a "happy" owner of a debian mail server with iredmail on board. In the morning I received a letter of happiness that my ip was blocked for spam. How and when this happened I do not know. I don't know where to dig to find the pest. I wrote to the office that the ban would be removed from me, but I still did not understand how to find the reason.
I will be glad to all advice