Other than for specific watch apps like Messages/Messaging, Health, StopWatch, Timers/Alarms, etc.., I view the smart watch concept as mainly a notification based system. That is, lots of notifications can be received (and cleared or not) at the watch, but that's only meant to be a window into what is actually on the phone.
I have never tried to respond at the watch to a received email notification, but for me the standard quick response notification options are there, whenever I get a received email notification. For me, normally the received email notification doesn't show enough of the actual email contents for me to ever want to use the watch to respond. I'll do a test and then report back, but with no actual email app per se at the watch, I don't see how I'd be able to get a window view at the watch for past emails received or sent, once a notification is cleared.
For messages/messaging (because those I think are generally shorter than emails), I think the concept is supposed to be opposite. That is, by opening the messages/messaging app at the watch, one gets a conversation view list of messages sent and received with/for each contact/number that one has been exchanging with. I have not double checked to positively confirm if the list of messages at the watch goes back in time forever, but that conversation view list for me seems to be exactly the same as what I see at the phone for all messages back and forth per contact, and that's how I would expect and want it to be. For me personally, I have no need to clear the list at the watch. For me, I use the clear (or not) notification feature at the watch, to manage what messages (notifications) I have or have not yet read or dealt with.