My server did not provide the correct return path in the source header when WordPress sent an email, causing me to be blacklisted.
I fixed this by adding this string to wp-includes / pluggable.php and know that this can be overwritten in updates, so I'll create a plugin for it later and just include it in the plugin
What I don't understand is that WP hasn't even got my fully qualified domain name. It looks like it got my hostname but not the full hostname -f full name. Force Instance Typically localhost.localdomain is a default host and WP has got virtual.localdomain and my FQDN starts with virtual. *
The email specified which email is in the general settings, but in the source header the email was created with the return path firstname.lastname@example.org.
The username was not even a user in WP. It was a Linux user that I use as an administration for this virtual domain.
This is for my own curiosity and understanding. Where does WP get this from? I couldn't track it …