The Postmaster Address
Any mail system that sends mail must accept mail for the unqualified postmaster, i.e., RCPT TO:<postmaster>, (RFC 5321 section 126.96.36.199), and a postmaster address for each domain it is responsible for. This provides a standardised contact point and mechanism for postmasters to communicate mail delivery problems amongst themselves.
There is some debate in the industry whether this should be a requirement, since some large mail server providers opt to disregard the RFC requirement in favour of obscure web-based contact / problem report forms, or private semi-secret contact points, which are frowned upon by many in the industry. Typically though, the majority of legitimate and responsible mail operators play well with others and adhere to this requirement.
While the postmaster address is intended to report mail delivery issues, it is sometimes used to report spam complaints and/or suspicious mail behaviour, though a separate abuse@ address is typically reserved for this purpose. Since postmaster@ and abuse@ address serve special reporting functions, they should not be subject to spam filtering as that would disrupt their ability to report spam evidence.