I use Outlook (currently Outlook 2007) and simply set my account settings to IMAP instead of POP3. I have around 15 accounts on 5 domains and it works fine. You can set Outlook to push all your email into one Inbox or you can keep the messages coming to the inbox associated with the domain.
You choose which account and domain you want to send new messages from. If replying to a message, Outlook defaults to the same account where the message came in at. Messages are kept on the server so you can have your secretary check your messages while on the plane. If you or the secretary deletes a message it's gone from the server, but you can choose an option that prevents one or more clients from deleting messages. You can have different signatures for each account and Outlook will default to the correct signature for the account you are working.
As someone mentioned above, one disadvantage is that when using IMAP your client will need to resync with the servers when you open your client and depending on how many messages you leave on the server, this may take some time. For most people this should be less than a minute on a high speed connection, even as short as a few seconds. However, if you never delete or transfer messages from your inboxes, you may wait very long time. In otherwords, good housekeeping is key to keeping Outlook or other IMAP clients nimble.
Outllook is full featured but can be a bit unsteady, at least it was pretty unstable in version 2003 and earlier. 2007 is stable, but not perfect.