Mail.app is crashing. In the window that reports the crash (and allows you to re-open) the specific problem Mail encountered is sometimes articulated in an inscrutable computer-speak message, near the top of the screen.
If you could write that down or cut-and-paste it here (just the text, please, not a photograph or screenshot) it may prove helpful. Those reports drone on for many pages that may be helpful for the developer, but they do not contain any useful additional information for debugging otherwise.
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Mail.app uses several complicated databases to keep track of potentially thousands of email messages, previous recipients, and email Sending and receiving Accounts. Occasionally, one or more of these gets messed up. Once it crashes, it is generally not useful to re-launch Mail until you have done a Restart.
The best way to proceed after such a crash is to invoke Safe Mode by holding Shift while you Restart your Mac. This does a five-minute Disk Utility (Repair Disk [directory]) that assures the File System Directory is in good order. Then it demands your Userid and password (even if you normally auto-login). Most extensions are not loaded, so you may find some things do not work properly (but they should produce error message, not just hang).
