Mail doesn't save Rules

Deleting Mail will do no good. No settings are stored in Mail.

Did you try to delete the UnsyncedRules?

Another option is to quit Mail then drag the com.apple.mail folder from ~/Library/Containers/ to your Desktop.

Start Mail. All of your settings will be lost, but your accounts and email should remain intact.

If that doesn't solve the rule saving problem, you can delete the new com.apple.mail folder and drag the one from your desktop back to the Containers folder. Quit Mail first.

Does it save if you use another account on your Mac. You can test in the Guest account, or create a regular account. You'll have to set up your email account on that new user account.

You can also try it while booted into Safe Mode. Use safe mode to isolate issues with your Mac - Apple Support

Safe Mode clears some system caches, so just booting into it may fix something.

Hello MalayTim,

Thank you for stopping by Apple Support Communities. It sounds like you're having trouble with rules disappearing after you've created them for Mail on your Mac. I'd be happy to provide information that may help.

You may want to start by isolating the issue. Try the steps below:

Try starting up in safe mode: Use safe mode to isolate issues with your Mac.

Also, testing in another user account on your Mac is a great way to further isolate the cause of this issue: How to test an issue in another user account on your Mac.

Finally, take a look at the steps here to make sure you're following the steps in the correct order: Use rules to manage emails you receive in Mail on Mac.

Take care!

MalayTim Said:

"I have no ~/Library/Mail/V6/MailData/SyncedRules.plist file, but there is an UnsyncedRules.plist file."

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To get this back, reinstall the macOS.

For Note: If you reinstall the macOS, nothing would be deleted. All intended, original files and Apps that came on your macOS would be reinstalled. It’s all about keeping your system files intact.

Important: Create a Time Machine backup of your Mac, so that you can have something to restore your Mac from, should anything go wrong.

What are the permissions on each of the folders in this path?

~/Library/Mail/V6/MailData

Library and Mail should both have your user as owner (r/w) and everyone no access. No group.

The rest will have a Staff group and everyone, both read only.

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

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:

Then we can examine the report and see if we can determine what's causing the problem.

MalayTim Said:

"I did a reinstall of MacOS. It did not fix the problem."

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Try This:

Rid of Mail first, then Reinstall macOS:

If you do not delete Mail, then Mail will remain the same, with all its settings intact. So, first delete Mail from your Mac. Then either:

• restore Safari from a backup; or 
• reinstall the macOS

Try this Link: Use AppleScript scripts as rule actions for Mail. So, follow it and make sure a rule is created and submitted successfully.

Try copying the required plist file from the new account to the same location in your old account and see if Mail will save rules then.