1. Start Outlook.
2. Go to your Inbox.
3. Click [New] to start a new mail message.
4. If the message does not show a “From” field then pull down the “View” menu and check “From Field”.
Note: I have noticed that if you have Outlook set to use Microsoft Word then the “From Field” does not appear in the “View” menu. The work around is to clear the option to use Word (Outlook → Tools → Options → Mail Format → Use Microsoft Office Word 2003 to edit e-mail messages), you will then be able to show the From field, and this remains even if you re-select to use use Word to edit emails.
5. Click [From…] and select the account that you wish to send an email on behalf of.
6. Compose the email as normal.
• When you try to send the email you will get an error if you have not been granted the necessary send-on-behalf-of permission.
• To the recipient the email will show both the true author and who it was sent on behalf of.

Think I got this from the MS site, can’t remember.