[exim] Open relay?

Hello,

one of my server with exim 4.77 seems to be an open relay, but I mean I had configured all right. I use smtp authentication and suppose, that hackers had found out username and password of a legal user. Is it possible to logging all information floating from mail client to server? I hope to get the username which give access to the server.

Or do you mean, there is an other reason for open relay?

Raba