"mail application is not open" on macbook pro laptop "

genijet wrote:

Cannot open mail from icon or from applications in finder

old issue, new issue, what changed— you say very little here...

To trouble shoot further you can:

—A SafeBoot Use safe mode on your Mac - Apple Support will sort many anomalies

Does a quick disk repair before it fully boots up, and certain system caches get cleared and rebuilt, third party system modifications and system accelerations are disabled.

Login and test. Reboot as normal and test. Caches get rebuilt automatically.

This test will tell you if third party interference; most* extensions etc are not loaded in safe boot mode.

—Test issue in another user (or guest user) account Change Users & Groups settings on Mac - Apple Support

This will tell you if it a universal issue or isolated to your user/admin account. 

unplug all third party peripherals when testing

Uninstall all third party apps that are Cleaners/Optimizers/VPN/Anti-Virus

all known to cause issues on the macOS