Here's what I do to accomplish that:
In Outlook Express ... for the forwarded addys I create "Message Rules" so that "email@example.com" goes automatically into a designated folder created for that particular forwarded email addy to keep all business (per domain/per dept.) and personal email separate and organized.
To send out email with the forwarded addy(s) appearing in the "From:" header, I then create accounts in Outlook Express but I DO NOT check mark the "include this account when receiving mail or synchronizing", because I do not need to access the actual mailbox (due to the forwarding) for "incoming" mail.
I fill out the "Outgoing" SMTP info, then under the "General" tab, I enter the forwarded addy's info in the "User Information" section. Thus, when replying to an email addressed to the forwarded addy, I just select that account in the pull down list to be used for the "From:", so that the email sent will then show/use the correct forwarded addy's email/domain to the Recipient.