This issue started to appear 2-3 weeks ago. It appeared on my three ‘smtp.mail.yahoo.com’ (aka, @cox.com) accounts when I was attempting to send a message. The ‘Permanently store this exception’ worked and the messages ceased. My other account: pop3.mailfence.com with smtp.mailfence.com began having the same issue except the “Permanently store this except” function fails to work. At first, only sending (smtp) was failing. Now, I can no longer even access the account without the message (included) being displaying. I am aware that Norton is involved from viewing the certificate information . Yes, I use Norton 360 and have for decades. I have, also, been on the TB support site (https://support.mozilla.org/en-US/products/thunderbird) and found similar issues. Unfortunately, none of them seem to address this specific issue. Thank you in advance for your time and effort. Roger