An Outlook Question

I have a question for any of you Outlook 2003 experts out there:

I have 2 accounts in Outlook that I send & receive mail with: my @tannock.net account & my @pencilneck.net account. For each, the account, the listed email address & reply-to address corrsponds to the domain its being sent through. However, whenever I send mail, it always appears to be coming from steve@pencilneck.net. Which I don’t want. I want all my personal correspondance to be coming from @tannock.net and all my business correspondance to be coming from @pencilneck.net. I’ve done tests by sending mail to myself from both accounts, and when I receive the message, it invariably says from @pencilneck.net. I’m assuming that everyone else has this when they get mail from me too?

Have I done something wrong? Or do I have to set something special to actually get it to pay attention to the domain I use to send mail with? Or is my install buggy? Or is Outlook buggy? etc. Any answers, links, etc, much appreciated.