OL2003 doesn't offer such a feature easily. You can have the email sorted on arrival with the email filters YOU get to define. But I see no "click-here" solution to get the sending email address to always be proper.
As a programmer I know it is possible to craft a Visual Basic Add-On to OL2003, but that's very expensive.
For now, sort the email and use the Send via which email system as you wish.
No, I don't know of any email client that does this.
Yes, you could do this with two XP accounts, but then that has its own issues.
Running MS Outlook 03 on XP. I have 2 email addresses coming from 2 different servers. Currently, both emails come into the same inbox. This is fine for incoming, however, not good for outgoing. I have two businesses and need to make sure that outgoing emails are sent via the appropriate business. Ideally, I would be able to have 2 inboxes, one for each business, both accessable in the same Outlook window - and each would send via different email addresses. If this is not possible, can I at least set some sort of reminder that tells me to select the correct sending address prior to sending? It is very easy to forget to change my sending address when sending many emails.