I have the exact same issue as described here: Apple Mail rules do not find the mailbox … - Apple Community
I have rules set up in Mac Mail to move emails I receive to different mailboxes in my Google account. They will work, but if I quit and restart Mac Mail it 'forgets' the mailbox and gives the note in the rules window, under 'Perform the following action' 'Move Message' to mailbox: "Mailbox ... does not exist."
Mac mini, macOS 13.2
Give this a try: boot into Safe Mode according to How to use safe mode on your Mac . Reboot normally and check to see if the problem persists.
NOTE 1: Safe Mode boot can take up to 3 - 5 minutes as it's doing the following;
• Verifies your startup disk and attempts to repair directory issues, if needed
• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)
• Prevents Startup Items and Login Items from opening automatically
• Disables user-installed fonts
• Deletes font caches, kernel cache, and other system cache files
NOTE 2: if you have a wireless keyboard with rechargeable batteries connect it with its charging cable before booting into Safe Mode. This makes it act as a wired keyboard as will insure a successful boot into Safe Mode.
Apple Mail rules do not find the mailbox to be moved
