attack pop3/imap/smtp

[michal krajcirovic <konference@kraja.net>]

Hello,
addresses the delicate problem :-) The customer has on our mailserver 
your domain, there about 10 mailboxes. The problem is that the 
connections are constantly on the pop3/imap/smtp 
random@customerdomain.cz. That sometimes appears the attack is normal, 
but this problem more than a month, for example, now over 12 hours we 
have more than 340 000 invalid login (or attempting to them) to an 
invalid address.

I installed fail2ban, but the problem persists, the majority of the IP 
request comes just a few.

Can someone have a solution?

m.