You may need to update the mail server value that you are using in your clients site, if the mail servers are specific to the domains then it would need to be updated for it to work correctly. This should be the first thing to check.
If you are getting to the thank you page then it seems that the code is executing alright. If the code is exactly the same and the mail server does not need to be adjusted you may need to speak to the host to see if there are any logs relating to the testing of the non working page.