[exim] Block Port 25 Signins

Hi, all,

I'm having a problem with hackers trying to break into my email server
with various userid's and passwords, I suspect on port 25 (but haven't
proven it). I do eventually block the IP, but not until after several
unsuccessful attempts.

What I would like to do is prevent logging in via Port 25 (I use a
different port for signing into the server), hopefully stopping these
antics in their track.

Can someone suggest how I can do this?

TIA,
Jerry