On Wed, Feb 04, 2026 at 04:12:26PM -0800, Robert L Mathews via mailop wrote:
> Some investigation showed that those messages already had a > "Delivered-To: <destination address on our servers>" header before > they ever reached our servers. This breaks Postfix mail loop > detection, which uses that header to tell if a message has already > been delivered [1]. > [...] > [1] <https://www.postfix.org/local.8.html>: "If mail arrives for a > recipient that is already listed in a Delivered-To: header, the > message is bounced." For the record, Postfix delivery agents other than local(8) don't necessarily enforce delivery loop detection. In particular: - The virtual(8) delivery agent prepends a "Delivered-To" header, but does not perform loop detection (because it is never involved in mail forwarding). - The pipe(8), smtp(8) and lmtp(8) delivery agents only do loop detection when the "D" flag is part of their master(5).cf command-line. - Unsurprisingly, the discard(8) and error(8) delivery agents don't do anything with the "Delivered-To" header. The above is no way a blame-shifting exercise. Systems that are not performing final delivery SHOULD NOT be prepending "Delivered-To:" headers. Rather, the intent is to post information that may be helpful to someone who wants to receive such messages on a Postfix system, via the virtual(8) delivery agent or perhaps LMTP (without a "D" flag). -- Viktor. 🇺🇦 Слава Україні! _______________________________________________ mailop mailing list [email protected] https://list.mailop.org/listinfo/mailop
