Send From Another Account in Microsoft Outlook
So you have several accounts set up in Outlook, and now you want to reply to a message sent to email@example.com from your account firstname.lastname@example.org. Simple enough. Or you just want to send a message from an account other than your default account.
Create your message like you normally would by hitting reply or forward or by creating a new message. Then, from the Accounts button, hit the dropdown and choose the account from which you'd like to send the message.
Caution: Outlook 2000 has the same functionality, but the dropdown appears right on the Send button instead of on the Accounts button, and you only have this facility if you have the Internet Mail Only installation. There is no Accounts button in Outlook 2000. Thanks to Jocelyn Fiorello, Microsoft Outlook MVP, who had to beat me over the head with this information twice in the newsgroups.