I'm managing a bunch of mail servers, the MX are postfix+amavisd-new systems while the mailboxes are on a qmail+vpopmail box. Yeah, we already plan to move everything to postfix but that's another story (and a long one) :)
Recently I started to see some incoming mails that enter my system with a "
delivered-to" (actually in lowercase!) line containing the recipient address. My qmail system sees those headers and bounces the mail thinking it is looping (and I am 110% certain it is not looping).
Those headers are far from my "
Received:" lines, seems like there's some kind of SMTP server (maybe actually a transparent smtp proxy?) on the sender's LAN that adds those lines.
I did some investigations and as far as I can tell
Delivered-To is not officially covered by any RFC, but it's a de-facto standard used to prevent mail loops. As such, it should not be added by any server other than the one actually delivering the message to the recipient's mailbox. This was also covered on StackOverflow.
If that is correct then those headers I'm getting should be considered a bug of the system that is adding them.
Who is right? Can I tell my users that it is a bug on the sending site and keep enforcing this policy, or should I try to do something on my side to let those messages in?
EDIT: just to clarify, I am only concerned with incoming mail containing a
Delivered-To header with an email address that is actually hosted on my system. IE, my system is the "authoritative mailbox" for that address.
EDIT2: I did some more investigation and as far as I can tell those headers gets added by some SMTP service on the sender's side. It is one of those all-in-one linux appliances, acting either as an internal SMTP (actually used by their MUAs) or as a transparent SMTP (where their MUAs think they're talking to an external server but 25/tcp traffic gets intercepted).