Outlook 2007 From: name spontaneously changed

I have a user who sent me a message at 10:45am on Friday:

From mpress@ohprs.org  Fri Feb 27 10:45:43 2015
From: "Mike Press" <mpress@ohprs.org>
To: "'Mark Foley'" <mfoley@novatec-inc.com>

That same user sent me a message from the same workstation at 6:33AM today:

From mpress@ohprs.org  Mon Mar  2 06:33:37 2015
Content-Type: text/plain;
        charset=us-ascii
Subject: Re: Getting Calendars on iPhone
From: OHPRS <mpress@ohprs.org>

All of a sudden, his name in his From: address has been changed from "Mike Press" to "OHPRS". Why?

Another user, same thing:

Today at 8:25AM

From dsmith@ohprs.org  Mon Mar  2 08:25:28 2015
From: "Dennis Smith" <dsmith@ohprs.org>
To: "'Mark Foley'" <mfoley@novatec-inc.com>

Today at 8:29AM

From dsmith@ohprs.org  Mon Mar  2 08:29:09 2015
From: HPRS <dsmith@ohprs.org>
Content-Type: text/plain;
        charset=us-ascii
Date: Mon, 2 Mar 2015 08:28:45 -0500
To: Mark Foley <mfoley@novatec-inc.com>

These are IMAP accounts running on Windows 7 and Outlook 2007. Only these two have had this happen. All other users in the office have their usual name in emails. I've checked the Outlook account and their actual name is listed. I've even deleted and recreated the 2nd user's account.

OHPRS is the first part of the public domain name: ohprs.org. Otherwise, I don't know where that would be coming from. It's not the name of the AD domain.

What's going on?