I have made an update to your Center.php page to suppress this value from being printed in the email message. The update was to add another entry to the begins with and includes sections. I just added a new entry for PHPSESSID in both of these. You can see this on lines 9 and 17.
I'm including the updated version of this file, please backup the current one and test it out with this one at webassist > email > templates > Center.php Let us know if it stops the value from appearing.