I connected my mailbox but authentication failed. What should I do?

If the provider sign-in does not complete correctly you will see "Mailbox authentication failed". Click "Add Mailbox" again, choose the same provider, and redo the sign-in, making sure you approve all requested permissions and use an account that has access to the mailbox you want to synchronise. For an existing mailbox that lost its connection, use the "Re-authenticate" button in Edit Mailbox instead, signing in with the same email address as before.