Message delay(external delay) of roughly 5 hours, 2nd occurance even after setting up rules for the sender(s)

I am specifically having an issue with these certain contacts that for whatever reason, always results in a delayed email delivery to my users even after creating rules that should always allow. Running a message trace results in a "Delivered (with external delay) this last time equated to a 5 hour wait and the first encounter was roughly the same. Although these rules may not pertain to the issue I am experiencing, I have yet to find a whole lot of information as to what my specific problem is. From research, it appears most of the time a representative asks you to send them your message header for them to analyze. If someone could reach out to me from Microsoft and give me a better understanding as to what I can do to resolve this, I would appreciate it.

Thanks.