Opus wrote:

For some reason, my Outlook Address Book thinks that a fax number is
a valid e-mail address.

It thinks a pager number is a valie address, too, and with good cause.
There are many tools with which Outlook integrates that allow sending of
messages by Fax or pager. Can't address a message if the address isn't
there.

How do I reset Outlook so that it doesn't have this issue?

You can't "reset" Outlook to not show fax numbers. What you can do, though,
is to use another field to hold the number, or modify the number in such a
way as to make it unrecognizable to Outlook as a fax number. For example,
prepending the letter "F" to it will cause it to not display in the Address
Book view. It will also cause it to be unusable by a FAX add-in, but if you
don't care about that, then you've achieved your goal.
--
Brian Tillman