We are aware of some reports of issues authenticating using TLS over SMTP since the installation of new SSL certificates on our mail servers overnight.
We have taken this issue up with Comodo as a matter of urgency (the reason appears to be a problem verifying the trust chain) and we hope to have a fix in place shortly.
In the mean time a simple work around is to disable TLS during SMTP authentication.
As soon as Comodo are able to provide a fix we will post a further update
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