A bit of an old post but I never found the solution for this message on this website. For me it was a problem of the user account on the mail server (which is the same host). The Maildir for the local user was missing, so the mail server (postfix) threw an error when the pop3 connection was established. I guess there is some bad try/catch in the logs that shows this as a “connect” error but it is simply a mail server error.