Fix Apple Mail crashing on El Capitan (Mac OS X 10.11)

This description assumes that you have an IMAP account or something similar that keeps mail on the server so that the Mail app can rebuild it locally. If you have a POP account, your email address will be lost.

I did this mainly to remind myself what to do if/when Mail starts crashing again on my mother's laptop.

Please take the opportunity to connect and share this video with your friends and family if you find it useful.