After I upgraded to Big Sur, in my mail application, I no longer have the mailboxes for saved e-mails-how to get them back?
iMac 21.5″, macOS 11.6
They are all there. They may be hidden. The way mailboxes are displayed changed slightly with Big Sur.
Where are these mailboxes supposed to be? In some of your accounts (IMAP, Exchange) or saved directly in your Mac?
Locate "On My Mac" in the Mail window Sidebar. When the cursor is near the words "On My Mac" you will see a little arrow like ">"
Click to twirl open and show the mailboxes.
Similarly, for mailboxes that are synced to your online accounts, like iCloud, Gmail, etc.
From one Senior ( me ) to another Senior - pleasure was all Ours
If the mailboxes are on the server, then the connection is probably IMAP.
Do you see the name of your account in the Sidebar? If you do, then proceed just as I described, but on that account instead of On My Mac.
If you don’t see it in the Sidebar you probably need to add the account in Mail->Accounts (which opens System Preferences ->Internet Accounts).
mailboxes no longer there -how to get them back?
