Reinstall or fix Mail (bug) Catalina 10.15.3

First boot into Safe Mode (Use Safe Mode to isolate issues with your Mac and Playing Safe: what does Safe Mode Do?) and check there to see if the problem persists. (This tells us if there's any 3rd party software that might be contributing to the problem. Reboot normally and test again.

NOTE: Safe Mode boot can take up to 10 minutes as it's doing some system cache cleaning, volume verifying and directory repairing. 

If the problem persists in Safe Mode try downloading and applying the macOS Catalina 10.15.3 Combo Update. 

Log into another user account, Set up users, guests, and groups on Mac,  and see if the problem persists. (This tells us if the problem is limited to your user account or is system wide).

If the preference pane is normal in the other account there's something in your account that's causing the problem. If that's the case download and run Etrecheck. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support  to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.

Copy the report

and use the Add Text button to include the report in your reply. How to use the Add Text Feature When Posting Large Amounts of Text, i.e. an Etrecheck Report

IMPORTANT:

Before running Etrecheck assign Full Disk Access to Etrecheck so that it can get additional information from the Console and log files for the report:

ALSO IMPORTANT:



Then we can examine the report and see if we can determine the root of the problem.

FWIW you can copy the text this way with the free version:

Then go to your browser and insert it using this button:

You have some unnecessary security and anti-virus software installed: Avast and MacScan. The are totally un-needed and will adversely affect system and application performance. Uninstall them according to the developer's instructions.

Macs do not need any "cleaning", "optimizing", "speed-up" or anti-virus apps. Read this user tip on what you need to do to protect your mac: Effective defenses against malware and other threats - Apple Community.   

Also go to the Clean Up section of the Etrecheck report and remove the orphaned files listed there.

You have an extraordinary large number of Internet Plugins. Make sure you need them all and remove those not needed.