Mail won't open

Hello ozdavey, 

Welcome to the Apple Support Communities! We understand that you'd like to know how to reinstall the Mail app on your Mac which is running El Capitan. 

Mail is part of the El Capitan operating system. To reinstall Mail, you'll need to reinstall El Capitan. While reinstalling the El Capitan operating system won't change the data which is on your Mac at this time, it's always recommended to have a backup of your data. Having a backup of your data is like having an insurance policy, you don’t plan on using it, but it’s great to have if you need it.This site will show you different ways you can back up your Mac: How to back up your Mac - Official Apple Support. 

You'll want to follow the steps for an Intel processor which are listed under, "Start up from macOS Recovery," in this article: How to reinstall macOS. When you go to reinstall El Capitan you'll see the menu will show OS X Recovery, and may look slightly different. Not to worry, this is due to the article showing what the menu now looks like in Big Sur. You'll see "Reinstall El Capitan," where it shows "Reinstall macOS Big Sur" in the article. 

Let us know if you have any questions. 

Take care.

You cannot reload only the Mail program. You can only reinstall OS X.

Mail won't open

The reason for that 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.

Hi ozdavey,

You're very welcome! Sounds as if you may no longer be experiencing the issue. If you are, let us know.

If the issue has been resolved, and you find you have any questions down the road, we're only a message away.

Enjoy the day!