Removing “On Behalf Of” from emails in Outlook sent by PHP and Sendmail in cPanel / EXIM
In the EXIM Configuration Manager - Basic Editor if you change the Set SMTP Sender: headers to ON it will pass an -f flag to Sendmail that will create “On behalf of” notices in Microsoft® Outlook, and can also help track abuse of the mail system since recipients will see the SMTP login used to send each message.
However if you turn it ON and then turn if OFF, the -f flag is not removed.
To correct this, you must remove "local_from_check = false" from /etc/exim.conf.local
then do the following to rebuild exim.conf /scripts/buildeximconf
and then restart Exim /scripts/restartsrv_exim