A big part of the problem seems to be that Thunderbird identifies the certificate as "mail.corp.redhat.com:993" when the certificate is "mail.corp.redhat.com"
Bug 1693244 Comment 1 Edit History
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A big part of the problem seems to be that Thunderbird identifies the certificate as "mail.corp.redhat.com:993" when the certificate is "mail.corp.redhat.com". The certificate at "mail.corp.redhat.com" retrieves fine, and verifies fine. But the additon of ":993" to it seems to be messing everything up.