Does this sound correct ?
Starting to set up a small web server on my Pi.
Installed Nginx, mySQL and php7
Testing using php to send mail ( code known to work on another online php server ) and found I need a mail server installed.
Dug around and found that Postfix is already installed - I guess that was added as part of OMV as I was able to send mails from OMV when that was the only thing running on the Pi.
Could not get php to send mails - nothing worked and ( completely clueless ) tried for hours to find a way to make it work.
Eventually I thought - maybe I should test the mail sending from OMV > System > Notifications just to check the Pi can actually send mail.
The 'General Settings' was disabled ( probably because I didn't understand the mails it was sending me anyway ).
As soon as I enabled it, a load of mails arrived on my PC from the php script. They must have been sitting in a queue somewhere.
So it appears that the 'enabled' setting in OMV, also controls the sending of mail from php scripts.