I believe this is a setting in the Outlook email account. Like you, we use Outlook which has an active sync facility and will allow you to send text messages from your outlook desktop.
open your outlook account mail on the s2. then hit the left button (next to the home button). Then the "more" option", then "account settings".
Scroll down to the bottom under server settings and you can take the tick out of the bit that says "sms sync".
That should sort it for you..