Cannot log onto incoming mail server (POP3)

Cannot log onto incoming mail server (POP3)

I've been unable to send email using Outlook 2013 POP/SMTP since Dec 24, getting error "Cannot log onto incoming mail server (POP3). Your server does not support the connection encryption type you have specified. Try changing the encryption method."
Email was working fine all year until Dec 24. No changes were made at that date.
I've tried changing the encryption method, but nothing works. Error affects all users here.
(Incoming email was also down for a couple of days, but is working again today.)