I am looking for a solution (using forms and scripts) that will enable
me to easily set the 'From' field in Outlook messages accoring to which
from/reply-to address I want to use. First, some background...

I send and receive e-mails for two domains, say domain1.com and
domain2.com. Until recently, this was done via two POP3 accounts, each
handling mail via the respective ISP's server. Outlook handles this
situation easily as the appropriate sender/reply-to e-mail address can
be specified via the account's properties. Adding to that a different
signature per account means that when replying to a message, Outlook
takes care of everything automatically. When creating a new message, I
simply simply select the cirrect account from the New button's
drop-down.

Now we have moved over to an SBS2003-based MS Exchange solution, and
all the above has become much more difficult. On the Exchange side, two
users have been set up - user1 and user2 - such that user1 handles
messages from ISP1 and user2 handles messages from ISP2. All user2's
incoming messages are auto-forwarded to user1, and I have my Exchange
account set up in Outlook to handle messages for user1.

Thus, all messages for domain1.com and domain2.com arrive in a single
Exchange mailbox, but ensuring I reply with the appropriate From
address and signature has become an error-prone nightmare. What I want
to be able to do, presumably via a mixture of forms and VBA scripts, is
to be able to auto-fill the From address (and, ideally, the signature)
according to whether I am replying to a message sent to *@domain1.com
or *@domain2.com. Also, it would be nice to have a toolbar button, say,
which will create a new message from user2@domain2.com.

If necessary, I can also change the way things are done on the Exchange
server if that makes things easier at the Outlook end.

I am an experienced developer, but have no experience developing
Outlook forms or VBA scripts. Any help you can give will be
appreciated. Thanks.