Email Sending Problem to One Individual

A coworker of mine is having an issue sending emails to one person. She is using Microsoft Outlook. She has been able to send emails to him no problem right up to about a week ago.
She has 2 email accounts, and he does not receive from either of them.
She has tried emailing him from her work webmail account, and he did receive that one.
I sent him an email, and he did receive it.
He checked him spam filters and his spam folder, and they are not going in there.

 

Are any of the parties involved receiving bounced messages with a bunch of technical gobbledy-gook in them (possibly attached)??

 

Nope. Nothing is being bounced back.