On Thu, Jun 11, 2026 at 05:21:32PM +0200, Victoriano Giralt via Postfix-users 
wrote:

> Relevant log line:
> Jun 11 13:18:46 frontmta postfix/smtpd[1111697]: NOQUEUE:
>   reject: RCPT from mail.example.net[192.168.0.2]:
>   550 5.1.1 <[email protected]>:
>   Recipient address rejected: undeliverable> address:
>   host backmta.example.com[172.16.3.14] said:
>   550 5.1.0 <[email protected]>: Sender address rejected:
>   example.com (in reply to RCPT TO command); from=<[email protected]>
>   to=<[email protected]> proto=ESMTP helo=<mail.example.net>

The verification probe sender address is chosen by the "verify" service,
which uses "address_verify_sender" and "address_verify_sender_ttl" (if
non-zero) to construct the probe envelope-sender address.

> [root@frontmta ~]# postconf -n
> address_verify_sender = [email protected]

Perhaps you have some master.cf settings that override the above main.cf
setting (assuming these are from the right Postfix instance).

> masquerade_domains = $mydomain
> masquerade_exceptions = root, postfix, double-bounce

The masquerade_domains setting is a form of wildcard rewriting that
is best avoided.  If necessary, rewrite specific valid addresses to
alternative valid addresses via a fine-grained canonica_maps table.

> virtual_maps = hash:/etc/postfix/virtual1 hash:/etc/postfix/virtual2

Best to use "virtual_alias_maps = ..." and to configure any
"virtual_alias_domains" explicitly in their own setting.

-- 
    Viktor.  🇺🇦 Слава Україні!
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