how to send e-mail from different adresses in exchange

Hi,

I own two companies and I have one windows 2003 SBS domain name. In exchange I receive mail for the two companies under two different mail domains. that is Oke..
But now I want to send mails one time for one comapny and the other day for the other company. problem is that with all my mail aliases setup in Exchange I cannot select in Outlook on behalf of which account I want to send.
Can anyone tell me if this is possible at all and then how to set it up?