I wrote:
It isn't just Postfix, many MTAs do that.
"To: arnt" isn't legal syntax today, but it was legal a few
decades ago and quite commonly used for years after the syntax
changed (in, IIRC, 1982) and various MTAs still contains glue to
support that. I think that's mostly a bug these days. It landed
on the wrong side of the fence when servers learnt to
distinguish between their own users (for whom they will relay)
and others.
To elaborate on that:
In the old days, mail servers didn't really have much of an idea whether
they were serving their own user or someone else. Then came the spammers.
Today each and every mail server is loyal to either the sender or the
recipient of a message.
Ones loyal to the sender know something about the sender, and are willing
to send to the entire net on behalf of that sender. Ones loyal to the
recipient know nothing about the sender, and are willing to send only to
their own (or whitelisted) set of recipients.
My point here is that when the sender isn't authenticated, then there is no
reason to believe that the sender is in any way tied to the MTA's own
domain. If an MTA (for whatever reasons) has to add domains, then it can
use an RFC2606 domain such as buggy-sender.invalid or
unknown-domain.invalid:
To: [email protected]
To: [email protected]
Those are safe and not likely to mislead any human readers. But gluing your
own domain name into mail from unauthenticated sources is bad, bad, bad.
Your own domain should only be used when carrying out the rewrites
mentioned in RFC4409, and that RFC only applies to authenticated senders.
Arnt
