My server was not providing the correct return path in the source header when WordPress is sending email causing me to fall on blacklists.

I fixed this by adding this string to wp-includes/pluggable.php and I know this could be overwritten on updates so I will create a plugin for it later and simply include this in the plugin

What I do not understand is WP wasn't even getting my FQDN, it seems like it was getting my hostname but not the full hostname -f full name. Force instance usually a default host would be localhost.localdomain and WP was picking up virtual.localdomain and my FQDN does start with virtual.*

In the email it would be set for whatever email was in general settings, but in the source header the return-path email was built as [email protected].

the username was not even a user in WP it was a linux user I use as administration for this virtual domain.

This is for my own curiosity and understand. Where is WP getting this from? I could't trace it...

I should also point out my env is this:

Centos7
Webmin version 1.940
Usermin version|1.780
Virtualmin version 6.08
mysql Ver 15.1 Distrib 10.4.11-MariaDB, for Linux (x86_64) using readline 5.1