Hmmm..
Check the domain's php.ini (/var/www/vhosts/domain.tld/etc I think)
; For Win32 only.
; http://www.php.net/manual/en/mail.configuration.php#ini.sendmail-from
sendmail_from =
[email protected]
; For Unix only. You may supply arguments as well (default: "sendmail -t -i").
...