Troubleshooting lost mail on a confirmed working form

I've asked similar questions in the past when universal email script isn't delivering the email, but in the past it as been an issue on the send side. I've now got a situation where I've confirmed the email is sending properly, as it works with two different "to" addresses, but on the address the client wishes to have the emails sent to, they never receive the emails. I assumed a spam filter issue, but they are having trouble locating it and the from address never receives any sort of bounce back either.

Just wondering if anyone has any specific ideas on how to troubleshoot further.

Also, does the script route mail through the host mail server? Sorry if that is a silly question, but I'm lost as to the specific functionality of it all and the email administrator on their end was asking about mail server administration.