Here's an article about certificates and what happens when they expire. It's written about Firefox, but it applies to Thunderbird, too. https://support.mozilla.org/en-US/kb/error-codes-secure-websites#w_when-you-cant-bypass-the-warning The issue can come for a few different reasons, one of which can be your anti-virus software. Otherwise, the problem is most likely to be a mis-configured email server. Unless you have personal control over it, there's not much you can do about it but to contact their tech support. Since the expiration date is today, maybe they are already working on it and if you give it some time and try again you may find it will have been fixed.