For what it's now worth, an IT Status news article has been posted:
On Mon, 24 Jan 2011, Mike Austin wrote:
> On Mon, 24 Jan 2011, Mickey Mossey wrote:
>> All my clients are running the highest version of Thunderbird -- 3.1.7 and
>> everyone has got
>> the error.
> The problem was due to an expired SSL certificate. It expired this morning.
> The certificate was replaced over the weekend, but the imap service didn't
> reload the new certificate on most of the imap servers, so it was still
> serving the now-expired cert.
> This has been addressed, and the new certificate is being served, so the
> errors should be gone for your users.
> Unless SAA tells you it's OK, please do NOT accept certificate errors - they
> are a sign of a problem, and a possible security exposure in general.