How are you connecting to the Internet ? I have a had this problem when using a library, cafe or other public access where all network requests are redirected to a self-signed SSL page to login to the network first (via web browser).

If you're using such a public access, make sure you login via the browser first, before accessing Thunderbird.

Even then some public access provider will block anything that is not web or web+SSL, which means POP and IMAP email retrieval/sync won' t work.

Thanks for your time and reply Fabian. Good point but no,this is my home pc-no-one else uses it. Also I have checked the TB Certificate manager for each of my 3 Gmail accounts,and all 3 show a supposed permanent imap.gmail exception registered,although only valid for the next 2 months as opposed to the others for some years to come! Yet that screen keeps on coming!

There has been a change in which file is used; cert8.db is now Obsolete. Previously used for Security certificates. cert9.db is now used for Security certificates.

Try this: In Thunderbird Help > Troubleshooting Information Clickon 'Open Folder' Button

a new window opens showing contents of your profile name folder.

Exit Thunderbird now.

Delete the file:

• cert8.db

REstart Thunderbird.

Is issue resolved ?

Hi. Thanks for your time and reply. From my very limited knowledge,your answer would seem to be a logical solution to the problem! I've done exactly as you said,so I'm REALLY hoping never to see that screen again! The next 48 hours will tell. I will confirm to you that it has worked (hopefully!) then. MANY thanks.

The most common cause of this these days is anti virus programs inserting themselves into the certificate chain. Is the issuer of the certificate your anti virus? Say McAfee, Avast Norton/Symantec etc

You never did post the information with your initial post, but I doubt it would have shown the issues that is shown when you click to "view details" or "View Certificate"

Hi Matt.Thanks for your time. I had hoped that the previous answer re cert.8 would solve the prob,but no! I have Avast Free/Avast secure Browser loaded,so I checked and the Cert Issuer is indeed Avast. I went on to their help,and it advised export the mailshield from Profile Folder back to Thunderbird,but when I tried to do that it just said the Mailshield was already installed. I've also gone on to Avast Forum for a potential solution. Should I uninstall the Avast-is there another AV which wouldn't interfere and cause this infuriating problem? Any suggestions VERY gratefully received!

I am not familiar with avast, but there are some notes from 2015 here https://wiki.mozilla.org/Thunderbird:Testing:Antivirus_Related_Performance_Issues#AVAST

That wiki has a link to https://support.avast.com/en-ww/article/91/#artTitle It is not just a matter of importing the certificate, AVAST is not a certifying authority, so there is a whole chain of trust that has to be built. At what point do the linked instructions fail?

Or you could just turn off the mail part of avast, be protected as much as you are now and side step this inconvenient bit of hacking.

Thanks again. I don't know at what point the instructions fail. I'm awaiting any results from the Avast forum,but nothing yet. I may well take your advice and just switch off the Mailshield....

Hi. Just to post for maybe anyone else who has this annoying problem. You were right Matt-the AV was the problem. The Avast forum advised a complete uninst/re-inst the Avast (from the Avast website uninstaller,not the usual apps) and that seems to have done the trick. Hallelujah! It was SO annoying. Hope this might help someone else solve the problem. Many thanks to all responders and best wishes...

Thanks for posting back and sharing the resolution to your problem!

If you have the time please mark Matt's post as the solution, this will help other with the same problem.

I have just concluded a free 30 day trial with McAfee and have downloaded AVG Free. I received the same 'Add Security Exception' notice. It said that 'This site ( i.e Thunderbird ) attempts to identify itself with invalid information.' At this point, I have not explored this myself apart from coming on to the Thunderbird support site which I have always found useful.

I really need Thunderbird to come up with detailed reasoning and some means of working round for AVG , please.

I quick response now that I have gone back to AVG. I started to enrol on their User Support site , made an error which necessitated me sending an email to them .... but the 'Security Exception notice' had blocked email use !! Taking notice from Dammit55 above , I went to uninstall AVG Free but in the process , AVG put up a notice asking why I was leaving and giving me two options either to Repair or Change. I opted for Repair and found that when I went back to the TB email , things were back to normal.

I will continue with my application for membership of AVG User Support and ask the question about Security Exception notices.