Score:2

Can an SMTP server tell the sending MTA to send to another address?

cn flag

Can e-mail forwarding be accomplished with the receiving MTA responding in some way that tells the sending MTA that it should send the e-mail to another address, or must e-mail forwarding be performed by the receiving MTA?

us flag
What is the actual problem you are trying to solve?
anx avatar
fr flag
anx
I have seen quite a few SMTP status messages telling the *operator* of the sending MTA to send elsewhere, but these are for *human* consumption, mostly the *"I think your system is malfunctioning, do this to fix it."* sort of instructions.
Hagen von Eitzen avatar
cn flag
By dropping the connection, an SMTP server implicitly "instructs" the sending server to perhaps try another (lower priority) host from the MX list held in DNS (but with the same address)
Speed Dial Dave avatar
cn flag
@TeroKilkanen Mail-forwarding on a "receive-only" e-mail server setup.
Speed Dial Dave avatar
cn flag
@HagenvonEitzen That's the kludge. Same address is not e-mail forwarding, which is what I was curious about.
Score:3
et flag

No. The current RFC (5321) doesn't list any response coes that instruct the sender to re-attempt to a different address. I doubt you'd want to trust that response even if it did. If the recipient can't be reached at the domain I specified, I would ignore any instruction that told me to connect to some 3rd party MX.

Addendum SMTP was built around unreliable networks with a strict process of hand-over. If the recipient doesn't explicitly confirm receipt from my sending MTA, then the message is still my responsibility. Add to this the equivalent of HTTP 302 redirects: What if I get one half-way through my redelivery schedule, does that reset the timer? What if I get a chain of them (or a loop)? How do I structure an NDR to the sender if there's potentially several failed recipients to report? It gets messy.

Speed Dial Dave avatar
cn flag
Understood. But I wonder about your response, "I would ignore any instruction that told me to connect to some 3rd party MX"... what counts as a 3rd party MX? MX records commonly point to a completely different domain than the one of the e-mail recipient - in other words, MTAs constantly connect to 3rd parties. If this is trustable, why wouldn't a "send it to someone else" type of response from the MTA in the MX record be equally trustworthy?
et flag
Fair point. I suppose I imagined that _end users_ of the receving MTA would be able to set such a forwarding address, which is a bigger risk. At least with an MX record, you know you're connecting to something that the _administrator_ of the domain was directing you to. I also have a technical concern I didn't elaborate on, which I'll add to my answer.
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