Mail

Apple hid the Users' Library folders...

Quit Mail first, then...

Method 1:

1. From the Finder, select the Go menu at top of the screen, and choose Go to Folder.
2. In the window that opens, enter ~/Library/Mail/V5/MailData/, and click Go.

How to rebuild the Spotlight index on your Mac

If searching your Mac doesn’t return expected results, rebuilding the Spotlight index might help.

1. Choose Apple menu () > System Preferences, then click Spotlight.
2. Click the Privacy tab.
3. Drag the folder or disk that you want to index again to the list of locations that Spotlight is prevented from searching. Or click the Add (+) button and select the folder or disk to add.
4. To add an item to the Privacy tab, you must have ownership permissions for that item. To learn about permissions, choose Help from the Finder menu bar, then search for “permissions.”
5. From the same list of locations, select the folder or disk that you just added. Then click the Remove (–) button to remove it from the list.
6. Quit System Preferences. Spotlight will reindex the contents of the folder or disk.

Might be this file...

/Users/YourUserName/Library/Mail/V5/MailData/FlagMailboxes.plist

Or you may need to rebuild the Spotlight index?

• Show flagged messages: In the Favorites bar, click Flagged, then choose a flag. Only messages marked with that flag are shown in the message list. To see all messages in the Inbox again, click Inbox in the Favorites bar.
• Rename flags: In the Mail sidebar, click the disclosure triangle next to the Flagged mailbox to show the flags (there must be more than one flag in the mailbox). Click a flag, click it again, then type a new name. For example, you could rename Red to Urgent.
• Search for flagged messages: Type a flag name in the search field, then select the suggestion in the Flags category. Or type “flag,” then choose the suggestion in the Status category.
• Clear flags from messages: Select one or more messages, click the Flag button in the Mail toolbar, then choose Clear Flag.

A flag mailbox is a type of Smart Mailbox. Learn more about how Smart Mailboxes organize messages.

If you use iCloud Drive, your flag names are available on your other Mac computers that have iCloud Drive turned on. For more information, see Store your desktop and documents in iCloud.

Any progress?

Great to hear, take care my friend! :)