It's a mystery to us why companies that send mail don't seem to care if its ever delivered.
Yesterday I dealt with mail from the domain: credit.trade.co.uk. Our servers refuse mail from this domain as it is invalid:
~# host credit.trade.co.uk
Host credit.trade.co.uk not found: 3(NXDOMAIN)
This is in accordance with RFC2505, Section 2.9:
2.9. Verify "MAIL FROM:"
The MTA SHOULD be able to perform a simple "sanity check" of the "MAIL FROM:" domain and refuse to receive mail if that domain is nonexistent (i.e. does not resolve to having an MX or an A record). If the DNS error is temporary, TempFail, the MTA MUST return a 4xx Return Code (Temporary Error). If the DNS error is an Authoritative NXdomain (host/domain unknown) the MTA SHOULD still return a 4xx Return Code (since this may just be primary and secondary DNS not being in sync) but it MAY allow for an 5xx Return Code (as configured by the sysadmin).
RFC2505 is entitled: "Anti-Spam Recommendations for SMTP MTAs", but the use of a valid return address goes far deeper than that. Anyone who sends mail MUST process bounces, and to do that their systems must be able to get them back in the first place. In the above case mail was refused before any delivery, so the admins at trade.co.uk will have seen the error if they were paying any attention at all.
Backscattering
MTAs,
such as unpatched
qmail
or unmodified
MS Exchange
will be unable to return a bounce message so outgoing mails sent there will disappear into a blackhole.
I emailed the trade.co.uk postmaster yesterday, so a fix may be imminent.