Smart Mailboxes

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What do you mean by disappeared? The menu item to create them or the previous smart albums that you had created? They are certainly still available with Ventura and Mail V 16.0.

The Smart Mailboxes are listed in the SmartMailboxes.plist file that's located in the Users/Home/Library/Mail/V10/MailData folder.

Copy that file to the Desktop and open with a text editor. Then do a search for a couple of the smart mailboxes you had. If you find none do the following;

1 - with Mail closed go back to the Users/Home/Library/Mail/V10/MailData folder and select it.

2 - go into Time Machine via the TM menu in the menu bar.

3 - go back in time to just before you noticed them gone.

Did this happen after a system update?

4 - select the MailData folder and click on Restore.

5 - When asked say that you want to save both the restored folder and the original folder.

6 - reboot and open mail. See if this will bring back the Smart Mailboxes.

Do you have any backups of your boot drive and user data? That's like driving without using the seatbelts. You don't need them until you do.

That would be the first order of business, get an external drive and start Time Machine backups. This drive from OWC (MacSales.com) is well suited for that. It should be 2 ½ to 3 times the size of the drive(s) you're backing up. OWC Mercury Elite Pro -HDD

I recommend drives from OWC (MacSales.com) as they have proven reliability, excellent customer support and warranties. 

If your. budget permits one of OWCs external SSD drives would be an even better solution but the HDD is perfectly suitable for the job.