I see. I will suggest that in the morning if they haven't gotten any further.

With one person, we tried his emailing to one of the set-up aliases for our coworker. For example, our standard emails are first initial, middle initial, and last name, though there is a workable one with the first name only before the @. This did get the email through from the outside person to the inside person, though when inside person replied, naturally it came from his main email, and then outside person couldn't just hit reply and get through. The inside person would have to remember to customize the Reply-To every time, unless there is a way to fiddle with the Outlook Rules to do that for him? I haven't sussed a way to make that happen.

With another sender, their IT is now in touch with our IT, so maybe we'll reach a solution. It's been a head-scratcher! Why just these few people have the issue, I don't know.