On Mon, May 18, 2026 at 10:21:32AM +0000, Pedro David Marco via Postfix-users
wrote:
> Hi,
> I am trying to understand the exact execution order between
> header_checks/cleanup and smtpd_milters.
> I have a custom milter that repairs invalid From: headers
> Example flow from logs:
> 1.- cleanup logs a header_checks warning on the original header From:
> <[email protected]>
> 2.- Then my milter replaces it with: From: <[email protected]>
> 3.- milter works ok, since postcat shows the corrected header in the queued
> message.
> What confuses me is that according to my reading of MILTER_README, smtpd
> milters run before queuing, so I expected cleanup/header_checks to see the
> modified header rather than the original one. However, the log shows that
> cleanup sees [email protected] and not [email protected].
> Am I misunderstanding how cleanup interacts with SMTP milters internally?
You failed to notice "milter_header_checks" in header_checks(5):
https://www.postfix.org/header_checks.5.html
header and body checks run first, then milters, and then
milter_header_checks. This could/should be documented more prominently,
perhaps in one or more of:
https://www.postfix.org/CONTENT_INSPECTION_README.html
https://www.postfix.org/BUILTIN_FILTER_README.html
https://www.postfix.org/MILTER_README.html
--
Viktor. 🇺🇦 Слава Україні!
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