my mailboxes disappeared

From your post in the other discussion, you said that when you hover the cursor over On My Mac, the button says Hide. If correct, then it means you On My Mac folder should be expanded already.

Go to the Finder menu and click on Go, then select Go To Folder and paste the path below in the box and hit Go:

~/Library/Mail/V4/

A Finder window will open to the V4 folder. You should have a folder for each account you created in Mail and also a folder for local mailboxes. You will need to look inside each folder an see if your local mailboxes show in any. If you can find them, then we can see about fixing Mail. If they don't exist, then you will need to find a backup where they do exist and Import them into Mail.

Below is an example of my Local Mailbox folder in my account. That is similar to what you will be looking for with your mailbox names.

He says in another discussion that his On My Mac section is expanded and not showing his mailboxes. He also has verified that the mailboxes exist in his library and they do not display in Mail.

Restoring data from TM will not fix the function of his mailboxes not displaying in mail. Apparently, the preference file for displaying local mailboxes is corrupted.

Did you do anything like update the OS? Check the App Store and see if anything got updated since the time they vanished.

Do all other mailboxes work OK besides the local mailboxes that have vanished? In other words, are the only mailboxes or messages missing the On My Mac mailboxes?

Do you have any backups of your system such as Time Machine?

Do you know how to access your user library?

Please answer all questions.

I've asked 6 questions in total and got a response to 2 of them.

!: Did you do anything like update the OS? Check the App Store and see if anything got updated since the time they vanished.

2: Do all other mailboxes work OK besides the local mailboxes that have vanished? In other words, are the only mailboxes or messages missing the On My Mac mailboxes?

3: Do you have any backups of your system such as Time Machine?

4: Do you know how to access your user library through the Finder?

OK. First off, since you are using Sierra, the V2 folder is useless. You can move it to the trash.

Follow the instructions below and see if it fixes your problem. Once completed, be sure to expand the On My Mac section to show the mailboxes.

If the first instructions of removing the container file does not work, try the second of reindexing mail. Please read very carefully and follow exactly all steps.

Remove Container file Mail:

Quit Mail.

In the Finder, select Go, then Go to Folder. Copy and paste the path below and hit GO:

~/Library/Containers/com.apple.mail

Move the whole com.apple.mail folder to the Trash, and open Mail and test. If all goes well, you can empty the trash. You may need to re create your user settings.

If the above doesn’t work, repeat the process with this folder:

~/Library/Containers/com.apple.MailServiceAgent

If there is a problem or you wish to restore the old file, first, Quit Mail.

Then open the Trash. Right click on the folder and choose Put Back from the menu.

Reindexing messages:

Quit Mail.

In the Finder menu, select Go, then Go to Folder and paste this path and hit Go:

~/Library/Mail/V4/MailData

In the Finder window that open to that folder in your user library, move all files with Envelope Index or ExternalUpdate to the trash. Open Mail and hit continue and let it finish. It may take a while if you have lots of email.

Restore from your Time Machine backup. Instructions are in Mail (Yosemite): Restore mailboxes and their content

Did you do anything like update the OS or did anyone have access to your computer that may have erased them?

Do all other mailboxes work OK beside the local mailboxes that have vanished?

One more important question: What OS version are you running?

Are you saying the whole V4 folder is empty? Can't be.

Your account folders should be there as well as MailData and RSS. Your folders will not be labeled the same as mine.