Wrong Name On Emails

Hello, 

I've been looking into an issue for a customer, they are sending emails and when the recipients get them it shows the sender information as incorrect, example: 

From: Dr Joe Smith <joe.smith@test.com> <matt.jones@company.com>

The second email that's showing is the correct one, but the name and first email address they have never heard of before (so not an old staff member, etc). This is happening to anyone he sends to, not just a specific person, so I know it's not on the recipient's end. 

They actually resolved this issue by resetting the user's password and having him sign out and back into Outlook. However, they now want to know why this happened, which is what I'm looking into. 

Any ideas?

Anything I've thought of so far don't quite fit the issue.