What does the mailbox status "Re-authentication required" mean and how do I fix it?

It means Deskhero's connection to your Google or Microsoft account is no longer valid, for example because the password changed or access was revoked, so mail sync has stopped. Open the mailbox (edit it) and click "Re-authenticate", then sign in to the provider again using the same email address as before. On success you will see "Mailbox re-authentication successful".