Just a suggestion...
From experience, I would suggest best practice would be to send your own order confirmation email and leave the official WorldPay email as is.
Maintaining the separation between merchant and processing gateway makes it easier when stuff goes wrong, its easier to see where the problem is. "I ordered apples, but the WorldPay email said I ordered a stack overflow" where do you start?
WorldPay do card processing, the email they send does the job but it aint exactly pretty. If you do your own email it can have the look and feel of your site.
Using universal email is so much easier than forcing WorldPay to do stuff its not good at. Its got wizzards!