SBS Exchange 2003 Question

PROLIANT

Hello guy's, another question but this time regarding SBS 2003 Exchange server, at the moment we have two companies abc.co.uk and 123.co.uk, all of the email is hosted off-site and we have a make-shift PC/Server running M-Daemon mail server, it collects the email from the off-site host via Domain- POP but I have been tasked with bringing everything back on to our Exchange server which has never been setup, I have only just started my new job as IT Manager and have inherited a mess and I am trying get a handle on it but need some extra tip's off the professionals..you guy's, so I understand that we will have one Active Directory called "abc.co.uk"
and I am familiar with changing the recipient policy for the SMTP server in exchange to allow multiple email domains, however my question leans more to the "out-bound" side of the email system so for those who need two email addresses do I create two user accounts in Active Directory for example "joe@abc.co.uk" with a matching email address for that domain then create a similar account in Active Directory and call it "joe-2@abc.co.uk" and then manually change the default email address to "joe@123.com" this will create the two separate mail boxes and then just add permissions to the mailbox so the person only needs to use the standard "joe@abc.co.uk" AD login.

Please excuse my long-winded question however I just want to make sure that I am doing the right thing here and I trust your professional wisdom to guide me in the right direction.