I have a corporate email account that uses Microsoft Outlook servers (but of course has its own domain). I just checked an email I sent from that account to myself on gmail, and it, too, has "namprdXX.prod.outlook.com" in the Message ID header field. It wouldn't be surprising if United used Microsoft Outlook servers for their corporate email services.

Of course, if you never actually requested something, maybe you should forward the message to [email protected] and ask what this is about.