It was so terrible after upgrading to El Capitan since the Mail App keep crashing constantly. If you have been facing this issue, take the following solution to fix completely the problem.
- Firstly, Quit the mail app completely, Turn Off the WiFi (or take off the internet cable) before reopen it again. Why? You believe that would prevent the mailbox from downloading new emails update data.
- Go to Mail ---> Preference ---> Accounts tab
- Disable all your mail accounts added (just uncheck the "Enable this account" box on all accounts). Then quit the mail app and Preference as well .
- Get your Mac accessed back to the internet, access back Mail ---> Preferences ---> Accounts tab and enable just only one account. You then should be patient to wait until the app no longer downloads new emails of the selected mail account.
- Do the same again on the rest of your email accounts.
After all above, the mail app will stop crashing.
Hopefully my solution does work on all of you guys.