My Mail app is always on the leftmost screen but hasn’t crashed from ages.
But, well, I only use 2 screens… 
I have 3
1920 x 1280, built in mbp, 2560 x 1440
if mail is on the 1920 then it crashes when I reply
its coordinates are, from what I can tell, negative offsets from the “main” mbp screen
if mail is on there when I reply it crashes
on the other 2 its fine
The animation made when replying is certainly taking a lot of resources and computation.
My second screen is also on the left (and, like any screen defined on the left of the main one in Mac OS, has a negative “left” value), but I don’t get a crash.
So the assumption that a negative offset could make the crash alone would be wrong.
A different OS version, maybe? Mine is 10.14. If you can start with another Mac OS version, give it a try.
But I’m guessing the hardware can be the problem, since a lot of resource is needed. Is your left screen connected to another output than the right one? And if you swap your two external screens, does it still crash on the same screen (the right one, then) or always on the one with a negative left value?