mail app quit unexpectedly

Sometimes a Safe Launch of the Application will fix this issue

With Mail closed HOLD the Shift key and launch Mail from the Applications folder

Still issues -->>

1 - Restart in Safe Mode. This will perform a Disk Repair, clear cache files and only load Apple Software, extensions and fonts. The boot up will be slow and can take some time - Normal.

2 - Does the issue present in this mode ?

3 - Sometimes a Safe Boot followed by a Normal Boot will just put things right.

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