"The certificate for mail.newcreationloughborough.uk is not valid for that server ..."

Please post a screenshot of the error. https://support.mozilla.org/kb/how-do-i-create-screenshot-my-problem

Hi christ1. Thank you for your offer of help. Apologies for not responding sooner, a bout of ill health has got in the way. I'm now sending you screenshots (with comments) of my attempts to add a new address to Tbird. Any advice you can give will be appreciated. The originating email address resides in Fasthost's web mail server. I hope this is of some use.

Click the 'View' button in the 'Add Security Exception' pop up, and inspect the certificate in the 'Certificate Viewer' window - see attached screenshot for instructions.

Please post a screenshot of the Certificate Viewer window showing the

• issuer,
• subject, and
• subject alternative name

fields of the cert.

Thanks for your reply. Things didn't quite go the same way this time when I tried to add the new account to TBird. Please see the attached file for what happened (the right-hand one of the pair). I think I've got the information you wanted in the first file attached (when I uploaded them, they arrived in reverse order of action, Doh!)

I can hardly read the certificate details in the second screenshot, it's too blurry. In any case, the issuer is Norton, which looks suspicious.

Try to start Windows 10 in safe mode with networking enabled. Then try again to set up the account.

Windows safe mode disables anti-virus software. This is a troubleshooting step, not a solution. Does the problem go away?

Apologies for the poor quality of the screenshot. I should have checked it first before sending. Just checking before starting up in safe mode, I'm using Windows 11, not 10, is that any bearing on the problem?. Is it really a good idea to connect to my ISP without any virus protection? Something could sneak in while I'm trying to synchronise the account. Norton is my antivirus provider, so I'm not sure why that should be suspicious. Before raising this query, I went through Norton's own procedure to download the certificate after it told me there was no certificate present. This is also EE Cybersecurity's solution (not surprising, as it is an OEM Norton Antivirus)

I'm using Windows 11, not 10, is that any bearing on the problem?

No, it isn't.

Is it really a good idea to connect to my ISP without any virus protection? Something could sneak in while I'm trying to synchronise the account.

Let's assume there is a phishing message sitting in your Inbox, with a malicious attachment. Nothing would ever happen as long as you aren't silly enough to open that message, and execute the attachment.

Norton is my antivirus provider, so I'm not sure why that should be suspicious.

Presumably Norton antivirus software, as part of its "protective features", is scanning secure (SSL/TLS/HTTPS) connections. This process involves intercepting the secure connection to the server, and and temporarily replacing the website's security certificate.

Alternatively to Windows safe mode you can first try to disable the SSL/TLS scanning or Email Protection scanning within the Norton Antivirus settings, particularly if you encounter certificate errors or connectivity issues.

Another possibility could be a problem with your email provider's cert on the server. Less blurry screenshots of the Exception prompt, as well as the certificate details would help.