This info is somewhat dated.

When using POP or IMAP accounts (ie. not with MS Exchange Server), you have to create another account in your Outlook configuration for the new identity. Then, depending on your Outlook version, you are presented with another 'From' address or an 'Account' selection to choose your 'From' identity.

1. Create a new Outlook account that is a duplicate of your original account.
2. Change at least the e-mail address (which also changes the Reply To address).
3. Configure the new account not to Send/Receive mail (it's actually the same account and will only complicate things if Send/Receive is left enabled) (Outlook 2010 shown, other versions similar):
   • Click Send/Receive (tab) → Send/Receive Groups → Define Send/Receive Groups → All Accounts → Edit
   • Select (highlight) the new account on the left
   • Deselect the “Include the Selected Account In This Group”
   • Save
4. When creating a new message, Click the From button to select the identity.
   • The “From” field is normally hidden. Expose it by choosing it from the Options menu when creating a new message.
5. Verify the clicking the Send/Receive button does NOT check the same account twice.

Microsoft Exchange makes identity management possible with Outlook.

See the properties “Send As” vs. “Send on Behalf of”.

http://www.webville.net/kb/Knowledge%20Base/Multiple%20Identities.aspx

http://www.howto-outlook.com/faq/aliases.htm

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http://www.ivasoft.biz/