Hmm. Intersting. Did this exact same thing twice in a row on my iPad Pro running iOS 11.4.1:
Tapped mail, Custom size 4000, picked a receiver, and tapped 'Send'.
The first time, it presented a "This message is 1.4 MB. You can reduce message size by scaling the image.." sheet where I could pick between a few smaller versions or the actual size - picked actual size and it sent it no problem.
Then I did the exact same thing again - and it never asked about the size, but just *looked* like it sent, but in my code I could see it returned a "Cancelled" status, and the Mail Composition Service crashed.
I'm thinking that maybe it could be a memory issue? That it takes up too much memory and just gets killed. The crash logs are sent to Apple though, so I'm sure they know about the issue.
I just tested this on my iPhone X running iOS 12 beta 8 - and it worked 5 times in a row ..but then again, that's an iPhone..