Right now I'm having issues with a Postfix configuration in Plesk that is not supported by Plesk and made by someone that is no longer available, I don't dare to change anything, even if I'm quite sure that it should work.
It is as follows.
I wanted to active PHP mail on our Plesk enviorement, after testing it I looked like i didn't work.
After debugging it showed that PHP generated the mail but it was not sending.
The logs showed that it was using Sendmail and trying to connect to a smart host that is no longer in service.
So I looked in the Sendmail configuration file and found in the config.mf file and I should be able to change the smart relay.
But what would happen? Could it be that I works currently and this is just a PHP issue? We have a mail node in our Plesk enviorement that i want to use.
You mentioned postfix, but then said you were using sendmail. We need a lot more information in order to help.