In addition to Dave's response, another option is to upgrade
to Windows Live Mail.
Windows Live Mail sets up separate folders for each account,
and if you are working within a given account folder, creating
a new message will automatically select that account as the
'From' identity. The only thing you don't get is privacy.
The Identities feature in Outlook Express gave the illusion of
privacy, but if any other person with access to your computer
wanted to read your email, it was easy to do.
