I heard of this Gmail-competitor MUA recently. https://superhuman.com/
Reading through its list of features, it seems quite easy to think of
scripting some of them, e.g. delayed sending by refiling into a to-send
folder that's posted in the background, or appending a future reminder
to a per-day file to be inc'd.
For the avoidance of doubt, this isn't suggesting we ship features like
this, just a reaffirmation that being able to script mail-processing
programs that work well together, and with the rest of Unix, is