Yes, that is correct. There is no reply in my Outlook (Work email) on Apple Watch.
But I can reply from within the Apple Mail App (personal email). I do not use Spark. I do use GMail, but there is no GMail watch support.
Additionally, by default Outlook for Apple Watch only shows focused inbox which sux. Took me a while to get it turn off and re-sync.
Honestly, I'm pretty underwhelmed with my Apple Watch 4. I got it b/c I always wanted one. And it is useful for me to see notifications etc. when I'm in the lab and cannot readily access my phone (like with gloves on). But honestly a lot of the notification capability exists on many other smarter watches and fitness trackers these days.