Yeah maybe not ISPConfig itself, but it NOT the script!!
these are prebuilt scripts like Joomla, not something I made.
My hosted domain is test.com, my server is on lab.com
The PHP script on the hosted domain sends an email using the PHP sendmail command, the user sending the email is firstname.lastname@example.org
so it is understandable that this will be the return path.
The from address is correct, email@example.com
, but when a mail server validates the sender, it validates the return path.
So 95% of emails being sent from this server work fine, but the ones that validate the return path will not accept.
This may be an unavoidable issue with hosting multiple domains, but maybe there is something that can be done? but if you don't know, don't tell me my script is wrong because it's not! you will just confuse people reading this.