You have been using Apple Mail for quite some time now. You have two accounts - one for personal emails and the other for business purposes. You have created many mailboxes in both these accounts. Until now, both your business and personal accounts were being displayed on the left side bar (in Apple Mail). All the individual mailboxes in both your accounts were also being displayed on the left side bar.
But today, when you open Apple Mail, you are encountering an issue. Only your personal mailboxes are being displayed on the left side bar. The mailboxes in your business account have just vanished into thin air!
You find that both your accounts are able to send as well as receive emails. All things seem to work fine except for the Apple Mail mailboxes disappeared on your business account, which have completely disappeared. You have searched high and low, but all your efforts have been fruitless. You cannot restore those mailboxes that have vanished from your business account.
You have been using High Sierra OS with Apple Mail version v11.5.
The solution to your problem is, indeed, very simple. Yet, it is very easy to be overlooked.
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