Use Tools>Options>Mail Format>Signatures and create a signature for each of
your accounts.  Use the "Select signature for account" drop-down on Mail
Format to associate each signature with its account.  Make sure a version of
the signature exists for each format type.  You must have a default
signature for each account, even if it's a signature file that contains
nothing more than a space.

Use Word as your mail editor and your signature for the account you choose
should appear in the message.  See if anything here helps:
http://www.howto-outlook.com/howto/signatures.htm