As required by section 4.5.1 of RFC2821, the RFC detailing the SMTP (Simple Mail Transfer Protocol), the postmaster alias (e.g. postmaster@domain.tld) is required on any system running an SMTP service. So far so good.
Having not gotten spam via my own SMTP, thanks to Greylisting I thought I was safe. But now spammers – scum as they are – resort to the very last method to spam valid addresses. Valid because they have to exist by the very rules that keep the internet alive! They “simply” spam postmaster aliases. Since the RFC requires to have as few obstacles in place as possible, postmaster was one of the few aliases that were whitelisted from greylisting (i.e. not subject to greylisting). Thanks to this new method I now removed the whitelisting, stretching “standard compliance” of my server …
… <your-favorite-adjective-swear-word-here> spammers!
// Oliver