Our non-profit organization (chgweavers.org) is running on Siteground.com. We have the free Let's Encrypt certificates as part of the service, which auto-renew every 90 days. Today was the day the old cert expired. Siteground claims that a new certificate for 90 days is now active. I have tried stopping and restarting Thunderbird, it still "sees" the old expired cert. I have even rebooted the Windows 10 system. Still no change. Obviously I could store and exception for the invalid credentials, but that is bogus. Have not been able to find how to kick-start/clear whatever Thunderbird so that it recognizes the new certificate. I even changed the mail server designation to Siteground's cPanel server, with its current certificate, and then back to see if changing server configuration caused it to pull the current certificate, which it did not.