As of the time of the writing of this post, there is no other information about this situation anywhere online I can find--and that's exactly why I'm writing this post! So hopefully, going forward, if you search for "Outlook Default User Nam" or "Default User Nam Outlook" or some other similar thing, you end up here. 2012-02-28 UPDATE: I can see that this post has already been indexed by some search engines and is appearing first in the results for the relevant searches. Great!

The problem:

When you create a reply to a message in Outlook 2011 for Mac, in the included message (that is, the part beneath where the message you're composing is, the part that shows the message that was sent to you, the message thread), where it shows your information, it shows your name as "Default User Nam" (or perhaps for you it is showing something different but still wrong; this post will help you too) in a manner such as this:

From: Kendall Adams <kendall@whateverdomainyouuse.com>
Date: Mon, 27 Feb 2012 08:30:00 -0800
To: Default User Nam <josh@whateverdomainyouuse.com>

See that instead of "Josh Adams" it says "Default User Nam" for me? Not cool. So why does this happen--and most importantly, how do you fix it? You'd think it would be as simple as changing the name associated with your email account; it's not. But I figured out what you need to do--read on!

The solution:

If you don't already have On My Computer folders turned on, do the following:

1. Click the Outlook menu and then choose Preferences...
2. In the Outlook Preferences dialog box, choose the General item in the Personal Settings section.
3. In the General dialog box, clear the checkbox next to Hide On My Computer folders (that is, make sure that checkbox is not checked).
4. Close the dialog box (use the red circle/x in the upper left corner).

Once you can see On My Computer folders, here's what you do to actually fix the problem:

1. Go to Contacts.
2. You should see My Contacts in the navigation view (the tree view) and one of the items in that view should be On My Computer; click On My Computer.
3. In the list of Contacts, you will see the culprit: a contact with the name "Default User Nam" (it might be the only thing listed, particularly if you're using Exchange because in that situation generally the rest of your contacts are kept on the Exchange server, not on your computer).
4. Now you either simply change the name on that contact to your name or, if like me you already have a contact for yourself set up, you can tell Outlook that the contact for you is in fact you by clicking on your contact then clicking on the Contact menu and then choosing This Contact Is Me. Note that after you do this, you'll want to delete the offending "Default User Nam" contact. Also note that Outlook won't let you choose This Contact Is Me for contacts on an Exchange server; however, you can copy your contact for yourself from the Exchange server to On My Computer: find your contact for yourself and hold down the alt key while you drag & drop it onto On My Computer.

If you have any problems with the instructions above, please let me know via a comment. I want this post to be helpful to people so if you hit a snag with it, please let me know so that I can help you work through it.