Escalated to Bugzilla from IssueTracker
For some reason, http://ocsp.verisign.net seems to be down/unresponsive today... our mail server's X509 certificate contains an OCSP URL pointing there, and thunderbird has an option to validate such certificates (Edit->pref->Privacy->Security->Verification).
Setting this and establishing a connection (IMAP or SMTP) then takes some time and comes up with the error message
"Error establishing an encrypted connection to smtp.cern.ch. Error Code:
Could you please provide a more descriptive error message in this case (since the encrypted connection to the server itself is fine, TBird is simpy failing to validate the certificate)?
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