I would. From your error message:
From the docs for xencart configuration, it seems you can set the script to not use sendmail:
zen-cart.com/wiki/index.php/Admin_-_Configuration_-_E-Mail_Options
Code:
SMTP Email Account Mailbox
(Admin - Catalog)
Enter the mailbox account name ([email protected]) supplied by your host. This is the account name that your host requires for SMTP authentication. Only required if using SMTP Authentication for email.
Default: YourEmailAccountNameHere
SMTP Email Account Password
(Admin - Catalog)
Enter the password for your SMTP mailbox. Only required if using SMTP Authentication for email.
Default: YourPasswordHere
SMTP Email Mail Host
(Admin - Catalog)
Enter the DNS name of your SMTP mail server. Only required if using SMTP Authentication for email.
Default: mail.EnterYourDomain.com
SMTP Email Mail Server Port
(Admin - Catalog)
Enter the IP port number that your SMTP mailserver operates on. Only required if using SMTP Authentication for email. Not used in 1.3.0.2.
Default: 25
This should be a more acceptable way to send email from your script, I think.
Not sure if that helps your issue specifically, but its certainly worth setting things up this way proper.