Hi zipstarman,
Thanks for using Apple Support Communities. Since it sounds like this issue is isolated to the Mail app, we'd recommend starting with the below guidance from If an app freezes or quits unexpectedly on Mac which can help with app specific issues. Since Mail is an Apple-developed app, some steps will not apply.
• If an app doesn’t quit as expected or it’s unresponsive, you can force quit the app (you may lose unsaved changes). Choose Apple menu > Force Quit, select the app in the dialogue that appears, then click Force Quit.
• If the app quit, reopen it by clicking Reopen in the dialogue that appears.
• Restart your Mac by choosing Apple menu > Restart.
• Make sure the app is compatible with your version of macOS.
• Open another file in the app. There may be a problem with the original file.
• Check for updates to your software.
• Disconnect any peripheral devices you just connected (for example, a printer). They may not be compatible with your version of macOS.
• Uninstall any app plug-ins or enhancements from other manufacturers.
If you recently updated the app or macOS, a plug-in or enhancement you installed may no longer be compatible.
• Diagnose problems with your Mac to learn whether hardware you installed, such as additional memory, is incompatible, improperly installed or defective.
Next, since this happened while trying to send an email, try the following from If you can’t send emails in Mail on Mac.
• Make sure the outgoing (SMTP) mail server is specified as directed by the provider of your email account. See Verify an email account’s outgoing server.
• Choose Window > Connection Doctor, then follow the instructions displayed in the Details column.
If there’s a firewall problem, verify that firewall software isn’t blocking email traffic on port 25, 465, or 587. Also check whether you’re affected by another firewall, such as software installed separately from macOS or built into an internet sharing router, or administered by your email account provider. Contact the appropriate person for information.
If the same continues, maybe the exact error message will help. What does the error message say?
Take care.