The default option for Postfix 2.2 and later is to never rewrite message header addresses from remote SMTP clients.
This problem may arise if in /etc/postfix/main.cf, "local_header_rewrite_clients" is set to "permit_mynetworks".
If mail enters the Postfix queue multiple times, via a content filter, spam filter, or some other external process, additional passes through the Postfix queue will originate from $mynetworks. Mail received from $mynetworks would then be mistaken as local and allow for empty domains to be rewritten to the local domain.
To resolve this, in /etc/postfix/main.cf, set "remote_header_rewrite_domain = domain.invalid". This allows Postfix to rewrite message headers from remote SMTP clients, and appends the remote_header_rewrite_domain value to incomplete addresses.
Setting this would then allow Postfix to append "domain.invalid" to an incomplete address as seen below.
Ex:
From: Postmaster <Postmaster>
-> From: Postmaster <Postmaster@domain.invalid>
Article ID: 126, Created: 4/23/2025 at 4:08 PM, Modified: 4/23/2025 at 4:36 PM