In your first post you said that when emails are sent you get an error, but didn't say what it was.
Then I asked what the error message was.
Then you said you didn't know. .... which is baffling to me because you said you got an error.
So then I gave you the above code changes so that the error could be logged to the /logs/ folder with any other PHP errors your site is encountering.
Now you can look in the /logs/ folder and find out what the errors are.
Ref:
http://www.zen-cart.com/content.php?...in-logs-folder