Outlook intercepts and disables it because of course it does and instead pastes it with the formatting but with a little drop down box at the end of your text pass address you can click to remove the formatting like you tried to do with you with hotkeys.

I have to use Outlook for work, but never intentionally do so if I choice. Truly amazing. If there's anything that can be messed up, it will be.

Also, the “Paste Without Formatting” button is paste without formatting