this has just happened to me on my imac - the gmail account has been set up for about 3 years - to transfer (Imap)all incoming emails emails to apple mail - suddenly it defaulted to POP - apple mail client shows: "The mail server denied access to the user specified in Mail preferences because of a serious system error. Verify the settings for account “admin@justus.group”. If necessary, contact your service provider to resolve the problem.
The server returned the error: Your account is not enabled for POP access. Please visit your Gmail settings page and enable your account for POP access." AND suddenly - the apple mail client account has been flooded with almost 2000 old emails -- HELP!