Resolution

There is a way to find what the folder the PHP script that sends mail was run from. Note, the paths can be slightly differ from the listed below depending on your OS and Plesk version:

1) create /var/qmail/bin/sendmail-wrapper script with the content:

#!/bin/sh
(echo X-Additional-Header: $PWD ;cat) | tee -a /var/tmp/mail.send|/var/qmail/bin/sendmail-qmail "$@"

Note, it should be the two lines including '#!/bin/sh'.

 

 2) then create log file /var/tmp/mail.send and grant it "a+rw" rights, make the wrapper executable, rename old sendmail and link it to the new wrapper:

 

3) Wait for an hour or so and revert sendmail back:

Examine /var/tmp/mail.send file, there should be lines starting with "X-Additional-Header:" pointing out to domains' folders where the scripts which sent the mail are located.
You can see all the folders mail PHP scripts were run from with the following command:
 If you see no output from the command above, it means that no mail was sent using PHP mail() function from the Plesk virtual hosts directory.