Mail App shut down

The cause is usually a malformed email message, and complaints are growing increasingly common. It seems to affect older macOS versions, exactly how old is uncertain though. Presumably newer versions of Mail can cope with them while older ones can't.

Press and hold a Shift key while launching the Mail app again. That will launch the Mail app with no message selected. In that manner you may be able to identify the problematic message, and delete it.

If you are unable to find such a problematic message because Mail keeps quitting, the only recourse you may have is to use your email service provider's webmail interface (assuming they offer one) to delete any likely problematic messages.

Hello MOG101,

Thank you for using Apple Support Communities.

We understand that you are not able to open the Mail app on your computer. It will open and immediately shut down. We would recommend starting by opening mail from the Applications folder and double-clicking the Mail app and not using an alias from the Desktop or Dock.

You can also try holding down Shift when opening Mail.

Let us know how it goes.

Best regards.