how do I stop sharing mail with other users on my computer

You may have to set up different user accounts in the computer if you wish to

continue sharing the computer among other people, and only use the Mail app.

Otherwise, if you use a webmail such as yahoo, gmail, or other, just use a browser

and login each time; then there is no saving of your email to the computer as such.

I've never used an email client software, nor had an ISP based email account. So

my numerous computers have never downloaded email into software. No loss.

But the answers do include setting up extra user accounts, not admin level, so the

other users do not mess with the operating system; a standard account or just a

guest account to use a browser to access web-based login email accounts, works.

There are third-party email client for OS X, some are free applications, others are

part of a larger browser product, such as SeaMonkey, & other Mozilla derivatives.

They provide an email segment or client aspect; unlike Firefox that's a browser.

Setup information should be within the Help viewer in the computer itself. Or by a

simple search online, including the OS X version and any email client software

your computer may have installed. Some browser software includes email client.

Also there are several ways to go in this matter. I don't see questions, only answers.

To learn how the Mail application works, if you want to import everyone's mail, then

figure how to set up user accounts in the OS X. Or use web-mail & login by browser.

Hopefully you can find the best solution for the unasked question.

Good luck & happy computing! 🙂