we have an SMTP problem. Our setup is a internal Exchange server and an external mail gateway (exim) with a DNS entry and MX record. This gateway accepts all mail for our domain and accepts relaying mails from our internal server and external staff (TLS and user authentication). This is the setup described and short words and it's working properly for years.
No we have problem with one of our most important client (staff >15k). They are rejecting e-mails from our external staff! Not because of a blacklist of our mail-gateway, it's because of a blacklisted IP in the
Received E-Mail Header. The e-mail is not sent directly to the target SMTP, of course it's relayed by our SMTP (which is not blacklisted there).
SenderX: IP (blacklisted at http://www.barracudacentral.org) --> Our SMTP Gateway: official IP, (not blacklisted) --> Target SMTP: accepts mail from our SMTP, but not from SenderX
In my opinion it's very unusual to reject mails because of checking the "Received" IP Adresses. The RFC states, they must be added.
RFC 5432: 3.7.2. Received Lines in Gatewaying
When forwarding a message into or out of the Internet environment, a gateway MUST prepend a Received: line, but it MUST NOT alter in any way a Received: line that is already in the header section.
What should I advise the postmaster of our client?