On 3/25/23 2:25 AM, Heiko Schlittermann via mailop wrote:
Ah, ok, that's what I know as MX sandwiching.
Interesting. I'll have to research that phrasing to see if I can learn more about it.
Ok, that was your point. Sure. We tried this (NoListing, MX sandwiching) for a while and had problems when *sending* messages.
Are you indicating that you had problems sending to others who were using NoListing / MX sandwiching? Or are you saying that your equipment had problems going through NoListing / MX sandwiching in your outbound infrastructure?
Some appliances (barracuda?) on the remote end implemented sender verification as callback, but where stupid enough to contact the 1st MX only (the one which did the TCP RST). As a result, they didn't accept our mails.
Well ... the idea is that a proper / RFC compliant SMTP stack is used ... which rules out some vendors. }:-)
I've never been a fan of Barracuda for a number of reasons.Would you mind elaborating how you tested NoListing / MX sandwiching? Did you 1) timeout connections, 2) send a TCP reset, or 3) send an ICMP error?
I've got an IP bound w/o a daemon listening, so the TCP/IP stack responds as if the SMTP service isn't currently running. -- I prefer that over a timeout as I think it's a fail hard ~> fail fast ~> move on to recover type sequence of events.
It's good to hear about others that have tried No Listing / MX sandwich and their experience therewith.
With our current greylisting implementation (using MAIL-FROM/RCPT-TO) as key, we didn't have issues so far. Until mailgun started (?) using variable senders for each delivery attempt.
I never understood different envelope senders for each attempt of a given message. -- I can see different envelope senders per message, a la. VERP. But I would naively expect each message to have a fixed envelope sender and recipient from submission time until delivery time.
-- Grant. . . . unix || die
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