To retain any customizations made to the file, Parallels Pro Control Panel does not regenerate the Sendmail configuration file after an upgrade. To take advantage of new service features or security fixes, you might need to regenerate the file manually after the upgrade.
To regenerate the file after an upgrade:
root
user.cp /etc/mail/sendmail.cf
<directory>/sendmail.cf.bak
where <directory> is the directory in which you want to copy the file.
/usr/bin/m4 /usr/lib/opcenter/sendmail/install/sendmail.mc > /etc/mail/sendmail.cf
/etc/rc.d/init.d/sendmail restart