Assuming you are referring to the address book view of one of the 7 versions
of Microsoft Office Outlook, you cannot sort that view by category. Sort the
Contacts Folder by Category instead, and send your messages from there. Just
select the Category > Actions > New Message to Contact...

Here's a thought. If your main concern is to avoid scrolling through a huge
contact list, I don't understand why you would have to do so much scrolling
in the first place, even with a very large collection of Contacts.   In most
versions of OL that I have used (1997, 2000, 2002, 2003) in the Address Card
view you should be able to simply type the first few letters of the person's
name and the Contact will appear in that view.   Actually, the first Contact
matching what you have typed will appear. You might have a few contacts with
similar names (e.g., Smith, Smithe, Smyth, Smythe, Rothman, Rothmann etc.,)
or even a bunch of people with the same last name, like a load of my own
relatives, and you might have to scroll through a couple contacts to get to
the right one.  But scrolling through hundreds or thousands seems quite
unnecessary, !.  When you want to send e-mail , you simply click on To and a
window should appear that lets you select a Contact by typing the first few
letter of the name.  Entering a Contact's address in Word is pretty much the
same thing.

Note 1:  This works best for very large databases if you arrange your
Contacts by Last Name first.  If you have them by First Name (for some
reason that is the default view), you may have to lots of scrolling ( e.g.,
finding "Richard Nixon" in a very large collection would probably be a lot
easier if you are looking for everybody named Nixon than everybody named
Richard or Dick)

Note 2:  Having different Contact lists (or even Address Books) leaves you
with the problem of trying to remember which Contact is in which list or
address book.  And if, as you seem to be saying, you have a whole lot of
them, how will you remember?  You might not be making things easier for
yourself!

Note 3:  I am assuming a "reasonable" sized OL Contact database, with maybe
up to a few thousand Contacts.  But if the number of your contacts rivals
the phone book for a very very large city,
than my suggestions here probably don't apply.  (And then maybe you need
something even more robust than Outlook!)