Apple Mail now crashing on opening; will not stop doing do

As of this morning. I'm using version 9.3 on an Imac running El Capitan 10.11.6. I'm updated.

Haven't had a problem like this before, and as far as I know, nothing's changed.

This seems to be a common problem, and I've followed various suggestions I've seen on the web. I've deleted "envelope index." Deleted .apple.mail.plist and com.apple.mail-shared.plist from the preference folder within Library Preferences (and oddly, they do not appear to have come back on restart). I moved the “com.apple.mail” folder to the desktop; no change. Restarting in safe mode makes no difference; it crashes instantly there too. I can't rebuild a mailbox because the program crashes instantly. I only have one email account.

I'm at a loss. Any suggestions?