I want to include an email address in reject messages, like "reject-error@mydomain.com", where users can write if they think the reject was not justified.

Let's say someone try to send me an email, and it's rejected by a rule in helo check, like "ACCESS DENIED. Your email was rejected because the sending mail server does not identify itself correctly".
I think in 99,9% the reject will be justified.
But imagine it's not.
I want this external sender to be able to contacft me on a special address, where this check will not be done, so he can explain why it's a mistake to reject his email, and I will be able to fix any issue in my helo_access file.

Hum yes this is a workaround but not really what I ask... but if it's impossible to specify to Postfix to not apply this check to a specific mailbox, that's probably the best solution.