Cotse.net is now requiring its customers use encryption to send mail.
From their user login page today:
Nov 16 – We now require encryption for the sending SMTP server (between you and us), if you are experiencing errors in sending, ensure that your mail client is set up to use STARTTLS or SSL/TLS.
The reason for this change is two-fold, first, if you want to send all your mail across an unencrypted connection, why are you using a service like us? Second, because frankly, we could not find definitive answers on the downgrade attack described here with regards to all email clients, specifically during an auto-config process. So, to just negate it, if the connection between you and us isn’t encrypted at all, the send will fail.
Another reason to love that company!