I'm seeing many people recommend Thunderbird. Let me enlightnen you.
I personally never trust any software that is not secure and private by default. Mozilla Corp is a for-profit corporation that makes nearly a Billion dollars in cooperation with Google monetizing data about your life. Thunderbird is Mozilla and if you setup with the Wizard, it already got the basics about your email life even if you disable it later.
Thunderbird Not Private by Default
- Sends all interactions with it to Mozilla
- Whether calendar is in use
- How many filters you have
- How many email accounts you have
- Computer/Device Information including hardware configuration
- Operating system
- IP address is logged
https://support.mozilla.org/en-US/kb/thunderbird-telemetry
Disabling Telemetry
- Click the menu button Menu Button and select Settings.
- Select the Privacy & Security panel.
- Scroll to the Thunderbird Data Collection and Use section.
- Deselect the Allow Thunderbird to send technical and interaction data to Mozilla checkbox.
Thunderbird Bad Security Practice of using a Primary Password
https://support.mozilla.org/en-US/kb/protect-your-thunderbird-passwords-primary-password
17 Criticial or High Vulnerabilies this year alone
Conclusion
If email security and privacy means a lot to you, or even computer security and privacy, your best options are to use BSD/UNIX/Void/Alpine and Claws-Mail. That is just the way the cookie crumbles in 2023.