- Postmaster address, defined
- The postmaster address is the
postmaster@mailbox every domain that accepts mail is required to operate, and it is the one address a server must not reject.
The requirement dates to the earliest mail standards and survives in RFC 5321. It exists so that an operator with a delivery problem always has somewhere to write.
In practice it is nearly always an alias forwarding to whoever runs the mail. Very few organizations staff it directly, and fewer still read it daily.
Because acceptance is mandatory, it is a poor probe target. A catch-all domain and a strictly configured one answer identically here, which is why the random-address probe uses an invented local part instead.
It remains useful for diagnosis. If a domain rejects postmaster@, its mail configuration is broken in a way worth telling someone about.
How ZapBounce reports it
Our catch-all probe uses a randomly generated local part rather than postmaster@, precisely because the required mailbox would accept regardless. Postmaster addresses submitted for verification are flagged as role like any other function address.
The same probe against two very different servers
Say you want to know whether acme.example is a catch-all, and you try the shortcut. You connect to its mail server and send RCPT TO:<postmaster@acme.example>. Back comes 250 2.1.5 OK. Then you try the same thing at strictco.example, which validates every recipient against its directory. You get 250 2.1.5 OK there too.
Now send each server a recipient that can't exist, such as RCPT TO:<zq7x4k2m9@acme.example>. Acme says 250 again, so it accepts everything, while Strictco says 550 5.1.1 User unknown. Only the invented address separated the two.
RFC 5321 goes further than most people realize. A server has to accept the bare word postmaster with no domain at all, as in RCPT TO:<postmaster>, and it has to treat the name as case-insensitive. It's the only recipient in SMTP with that special status.
When writing to the postmaster is the right move
Rejection messages often point you there. Text like 550 5.7.1 Message rejected, contact postmaster@partnerco.example is a genuine invitation at small and mid-sized organizations, where a real administrator sees that mailbox.
Write the kind of message an admin can act on in five minutes. Include your sending IP, the full rejection text, the time with a time zone, and one affected recipient. Skip the history and the frustration, and send it from an address outside the blocked domain, or it'll be rejected like everything else.
At the largest mailbox providers, the address isn't how problems get solved. Google, Microsoft and Yahoo each run sender support pages with forms, and a message to their postmaster mailbox is unlikely to get a human reply.
Your own deserves a test as well, so mail postmaster@yourdomain.com from outside and see who gets it. If the answer is nobody, create the alias today and point it at whoever runs your mail.
Postmaster address: common questions
Must every domain have a postmaster address?
Every domain that accepts mail, yes, under RFC 5321. It is one of the few hard requirements in the standard.
Can I use postmaster@ to test a domain?
Not usefully. It is required to accept, so a 250 there proves nothing about other addresses.
Does anyone read it?
Usually it forwards to the mail administrator. Attention varies enormously between organizations.