El Capitan Mail repeatedly quits unexpectedly

Hello!

I had this same issue just a few days ago on my iMac running OS X El Capitan. The problem, for me at least, is that this older version of the Mac operating system isn't compatible modern email message(s) you may have in your Inbox. To fix this, I signed into my Mail provider's online mail interface (since I have iCloud mail, I went to https://www.icloud.com/), and I moved all my email in my Inbox into a new mailbox folder I titled "Older Message [Date Range]". This solved the problem for me.

If this doesn't work for you, or it isn't feasible, you could always use your Mail provider's online mail interface full-time instead of Apple Mail. If you do this, I'd recommend using Google Chrome, since it's the most up-to-date and latest supported browser for OS X El Capitan.

However, if your Mac is new enough, you may be able to upgrade to a newer version of macOS and you won't have to worry about this issue. To see if you can, please respond with what year your Mac is:

Find the model and serial number of your Mac - Apple Support.

Jack

I have noticed of late that the very old version of Apple Mail has a lot of problems now with certain mail services.

Especially ones that use newer email ID verification codings today.

Many times, for me it takes the Mac OS X 10.11.6 version of Mail an inordinate of time to access my email services and many times I was getting an initial crash before my email would, eventually, load.

I finally gave up on that version of Apple Mail and started using a version of Thundebird Mail still compatible with MacOS X 10.11.6 El Capitán and this has been working much better than that very old version of Apple Mail

That old version of Apple Mail just is no longer able to handle the newer, updated email ID verifications now being used in many new and existing email services.

Found here.

https://www.thunderbird.net/en-US/

Best of Luck to You!