|This issue appears to have nothing to do with the user. |
If mail isn't getting to your server from the ISP's mail server the break is between those two not between the user and your mail server.
Usually if you have your own mail server you do not have your ISP hosting your mail and then transferring it to your internal server. Doing so causes delays in delivery which is unacceptable to most.
Are you sure of your configuration? Perhaps you are confused as to what the ISP is providing concerning mail? A dns mx record at the ISP does not mean they are hosting your email.