Re: Can my EdgeRouter help to protect against this kind of attack?

You can also use rate-limiting rules in your firewall.  This for SSH brute force attacks:

rule 4 {
         action drop
         description "SSH Bruteforce prevention"
         destination {
             port 22
         }
         log disable
         protocol tcp
         recent {
             count 3
             time 600
         }
         state {
             established disable
             invalid disable
             new enable
             related disable
         }
     }

You could adapt it for mailserver by changing it to refer to port 25 (and any other ports your mailserver uses/is being attacked on).