All this is specific to domains with no incoming emails at all and with no available MX.
A domain may not have any MX record, that's RFC-compliant, but may have a A record that doesn't point to any email server.
In this situation, postfix tries to deliver an email to this domain using the @IP from A record.
As the server at @IP is not a SMTP server delivery always fails (after a long time and a lot of retries).
The consequence of this situation is that postfix sends regular warning emails to the sender (temporarily failure) and a hard bounce message (undeliverable) after several days.
A good example of this is a domain such as gmail.fr. Of course, Gmail never created any email box @gmail.fr, however some eyeballs make a mistake and write emails to [email protected] instead of [email protected].
gmail.fr has no MX record but a valid A record for web redirections (no smtp server here).
What I try to achieve is to make postfix accept any email for *@gmail.fr and then send an undeliverable email back to sender at once.
The incoming email should not be rejected because my postfix server is a relaying server. It has to accept the incoming email and deal with it "later".
Edit: As suggested, an explanation on accepting emails on the relaying Postfix.
Imagine an application that create an email. This application has to drop this email on internal/intranet postfix that only take care of it by relaying it to the main postfix connected to the internet.
The application creating the email does not have any idea about email recipient validity and it does not care about it.
The internal postfix is only there to forward the email. It has no internet connectivity and all it knows is a smarthost.
The relaying external postfix gets the email and if something is wrong should bounce it to the sender (should not refuse incoming email).
