Mail unexpectedly quits in High Sierra

Safe Boot from the HD, (holding Shift key down at startup), does the problem occur in Safe Mode?

Safe mode attempts to repair Disks & clears lots of caches, so if Safe Mode works try again in regular boot.

If still no good Disconnect from the Internet, then start Mail, look for the last rec'd eMail.

Does Mail crash if no internet?

Quit Mail & login to your WebMail in a Browser, if possible delete any later eMails there after the last rec'd one on the Mac.

I've seen others with the problem, have cured several.

Safe Boot, (holding Shift key down at startup), does the problem occur in Safe Mode?

Safe mode attempts to repair Disks & clears lots of caches & loads safe Drivers, & prevents loading of 3rd party extensions, so if Safe Mode works try again in regular boot.

You can save us asking dozens of questions by submitting a report from this App for us to look at…

EtreCheck is a FREE simple little diagnostic tool to display the important details of your system configuration and allow you to copy that information to the Clipboard. It is meant to be used with Apple Support Communities to help people help you with your Mac. It will not display any personal info.

https://www.etrecheck.com/

Pastebin is a good place to paste the whole report if you capture the URL while there…

https://pastebin.com/

Whew, they've changed pastebin & made it harder, but after pasting in, click Create new paste button, then Embed button, then copy the URL...

<script src="https://pastebin.com/embed_js/KuvnghqA"></script>

The relevant part would be like...

https://pastebin.com/embed_js/KuvnghqA

Don't use these, they are just examples, yours will appear different.

Workable but harder for me to work with...the Note tool on the bottom of this editor's toolbar, as shown in the image, to copy and paste the output from EtreCheck. In a Reply before you click post, look for this to add longer texts...

I had the "mail quit unexpectedly" problem for ages on a late 2011 MacBook pro. Apple expressed no interest in helping . I tried all of the above fixes and more on other websites. I even did a complete re-install of High Sierra, but the problem persisted. What actually fixed it for me was using the facility in Mail to rebuild the database. I did that for each database, including sent, trash etc. And the problem disappeared! Much simpler than all of the previous methods - and successful.

Actually, although the problem want away for a few days, it recurred; there having been no changes to the system in the interim, and happens daily. This is after trying every single suggested fix found on this and other sites, and reinstalling High Sierra from scratch. I guess I just have to live with it.

Mail still working great here in 2011 iMacs 10.11.6 & 10.13.6.

OK, anxious to hear good news my friend! :)

make sure you rebuild every database, not just the inboxes. i.e. sent, trash, drafts and any others for each account.