Mail not delivered from custom email server (MS Exchange)
Solution 1:
Adding this as an answer : If your mail server is getting a successful SMTP message out, and recipients aren't receiving it, you must ask the people who own the next-hop mailserver. There is literally no other way for you to find out what that mailserver did with what your server gave them. You can guess all day long, and you should (in general) follow best practices and check if you're on DNS RBLs, etc, but it's still just guessing.