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.

The email goes through whatever is set up as the default SMTP server by the hosting provider.

Usually you can see emails that are spam blocked in a quarantined folder or spam folder. Sometimes the issue can be with the FROM setting or content of the email... sometimes it can be configuration settings of the server itself.

Look up DKIM and SPF records... those are server settings that can be configured that will improve deliver-ability of emails.

Your reply would seem to indicate the only possible problem would be a spam filter.

Am I correct and believing that if the emails are successfully sent to alternative email addresses, the issue would have to be a spam filter issue on the receiving end. The 'from' address isn't receiving a bounceback, so I would think the mail is delivering somewhere, but they are claiming it's not showing up in any spam filters.

I can't say for sure, but that is the most common issue. The fact that it sends to some email addresses means that it likely is a server issue somewhere, either the outgoing or incoming SMTP error.