Hi Felix,

On Tue, 25 Mar 2025 at 02:38:55PM +0100, Felix Oberst wrote:

here are both exports of postfix configuration.

thank you.

Guess I'll sent it over to the mailcow bugtracker then.

Better not, IMHO it works as designed. ;-)

smtpd_sender_restrictions = reject_authenticated_sender_login_mismatch,[...]

Postfix is configured to check if <us...@company.com> is allowed to send mails as <external_u...@gmail.com> (to <us...@company.com> in this case). And the default setting says: no.

As Christian already wrote: it's not allowed to change the sender address (event owner) of appointments - they have to keep the original address. And so, as intended, Postfix rejects such mails.

If tolerable you can "disable" this check or more precisely: allow <us...@company.com> to send as * in the settings of it's mailbox. Please have a look at the documentation:

https://docs.mailcow.email/manual-guides/Postfix/u_e-postfix-disable_sender_verification/

HTH and regards,
Markus

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