I installed exim4 on a Debian server configured it to use Dovecot LMTP delivery and everything works nice. But I'm having problems with a bit of a spam attack right now. I installed fail2ban but it's a bit slow to catch up. Also I was looking at the actions for the exim4 jail and I saw that there can be error messages with 535, Sender verify failed and unknown users and I think all of those sound like better ban reason than the current relay not permitted:
2015-11-23 09:03:25 H=118-160-211-95.dynamic.hinet.net (xxx.xxx.xxx.xxx) [18.104.22.168] F=<email@example.com> rejected RCPT <firstname.lastname@example.org>: relay not permitted
So I wanted to ask if there is a way to force authentication based on the fact that this is not even an mail server that I am responsible for (163.com and I'm responsible for example.com only) and to give different error message (like not auth)?
Also as a side note would this mean that I somehow forgot to add some configuration somewhere like the lack of auth in some ACL? (Every actual person from example.com needs to enter the real password before he sends an email and if wrong it comes up with an error message saying wrong password).