I am creating an HTML email to be sent to a user. If there is a valid email address to within an HTML email, Outlook will render it (normally helpfully) as a mailto: link.
Navigate to Tools > Options > Mail Format > Editor Options
Click on Proofing
Click on Auto Correct Options
Click on Auto Format tab and uncheck 'Internet and Networks path with hyperlinks'
Clikc on Auto Format as You Type and uncheck 'Internet and Networks path with hyperlinks'
Save and changes
(This solution is for Outlook 2007)