Issue configuring reply by email: 553 5.7.1 Sender address rejected: not owned by user

Hello Jay,

thank you for your quick reply,

Jay Pfaffman via Discourse Meta meta@discoursemail.com writes:

Your mail server doesn’t like the address. You need to figure out how
to allow the user that is sending mail be allowed to use that
address. It may be that it doesn’t like the + addressing? But I
suspect it’s that it’s denying forumreply.

Thanks to your reply I can now have a look to the relevant email headers
meta.discourse.org used in notification email sent to me:

Return-Path: <meta+verp-7f805e7e715078262ca96f39e643111c@discoursemail.com>
Message-ID: <topic/119897/590490@meta.discourse.org>
In-Reply-To: <topic/119897@meta.discourse.org>
References: <topic/119897@meta.discourse.org>
 5.7.1 Sender address rejected: not owned by user

So it’s clear that the meta.discourse.org instance is using the same
email address both for the notification email and the reply by email address template: am I misreading the headers?

In other words it seems to me that meta.discourse.org is using

reply by email address: meta+%{reply_key}@discoursemail.com

This means that the Return-Path is the same of From: this is why any
authendicated SMTP server will never complain about this.

In our instance, instead, we cannot use

reply by email address: forum+%{reply_key}@HIDDEN

since
Notification email cannot be the same as reply by email;
in fact I get a validation error if I try:

reply_by_email_address: Value must contain '%%{reply_key}' and be different from the notification email.

(why the double %%?)

If I use

forum+%%{reply_key}@HIDDEN

(note double %%) our Discourse instance is able to send the email via
our SMTP server, but the headers are different from the ones I
observe from meta.discourse.org

Return-Path: <forum+%verp-9b32231ead642e0cb2cb33563aaf413b@HIDDEN>

Please ho can I set our instance to have the same behaviour as
meta.discourse.org regarding reply by email delivery?

To be clear, I need to understand how to configure things in order to
have this set of email headers:

Return-Path: <forum+verp-7f805e7e715078262ca96f39e643111c@HIDDEN>

the same way meta.discourse.org instance is able to generate?

Thanks for your support.
degrees380