As soon as I got a solution against comment spam, the mailserver is flooded with junk. Spamassassin, my mailfilter is working great, but it is a waste of ressources to analyze every bit knocking at my mailserver. What I want to do is taking load off the filter process.
First of all, regarding the smtp protocol, the other mailserver should greet us with its FQDN (his name and domain). If this is not the case, I will refuse taking his mail.
The second action is a bit more complicated – greylisting, the topic this post should be about. Greylisting is temporarily rejecting any email from a sender the server does not recognize.
As mailservers are working according to the SMTP protocol, they usually retry delivering the mail a bit later. The mail is accepted then and delivered.
Spam and malware are usually not sending via common SMTP services, not retrying to deliver the messages. This way, their messages are kept away from our server.