It’s controlled by the Autoformat as you type tab, Border lines option. In Outlook 2010 and up, go to File, Options, Mail, Spelling and Autocorrect. To disable this feature in Outlook 2010 and up, go to File, Options, Mail, Spelling and Autocorrect. If you hit Ctrl+Z as soon as it changes, it will undo the line. Along with double underlines, it also inserts full lines for dashes (-) and underscores ( _ ) anytime you type the character three times in a row then press Enter. Selecting all text before and after and hitting delete moves the line. This line cannot be selected, formatted, or deleted. Typing = and then hitting return, Outlook puts in a double line.
