How do I disable macOS Mail.app and use only Thunderbird for handling email
You can simply disable Mail.app auto launching at startup and do not need to remove the email account from Mail.app. But if you wish to change your email client, read along.

There are two parts to answering your query:


Removing your email account from Mail.app.
Stopping Mail.app from automatically launching when logging into macOS.


You wish to use Mozilla Thunderbird as the choice of your email client. That's perfectly okay. You can use just Thunderbird, macOS Mail.app or both simultaneously to handle your mail (slightly depending on email protocol)

Here the answer for your question one-by-one:


If you do not wish to use email account(s) with Mail.app, simply remove/disable the account(s) from Mail.app. With the Mail.app open, go to Preferences → Accounts in the Menu Bar and disable/remove the added account(s).

Note: If your Mac is part of a company/enterprise and a system administrator has set it up for you, it is advisable to seek his assistance to remove the account and reconfigure the email with Thunderbird. Certain email protocol (POP3) may lead to data loss. Especially important if you are not familiar with what protocol your email client is configured.





Stop Mail.app from automatically launching. Go to System Preferences → Users &amp; Groups → and select your user account in the left sidebar. Click on the Login Items tab, select Mail app in the list and hit the minus button shown in the bottom.




You may still choose to leave Mail app in the list and check the box ahead of it to let and launch on startup but keep it hidden from view.