(For reference: this is about POP settings, not IMAP.) Click the Advanced button below the settings above in the "Message Storage" section and review the ''When downloading mail from this account's server, use the following folder to store new messages:'' settings. - Is ''Inbox for this account'' selected, and does the folder really/still exist? - If the second option (''Inbox for different account'') is selected, make sure the path is valid and the ''Include this server when getting new mail'' check box is ticked.