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I have 4 emails in Thunderbird, one of those I also have on my phone, it is receiving mail, on my desktop, two of the emails have no email today, and two I have only one email, all should have 10 to 20 or more emails. I do have the emails on the website, but they are not loading onto Thunderbird. I get this message, "The certificate for imap.knology.net is not valid for the server. Someone could be trying to impersonate the server and you should not continue.

 Can you offer any help?

I am getting an error for an outdated certificate on my mail smtp server and when I try to add a security exception Thunderbird does absolutely nothing when i click the button to add a security exception where I can send mail. I end up having to cancel the window and discard my email. It will also not go view the certificate and if you ask it to check it just hangs up forever in and endless loop of checking.

Hi,

I’m using Thunderbird on Windows and recently ran into a certificate warning that I can’t safely interpret, even after searching the web and using AI tools (which gave me inconsistent answers).

Thunderbird tries to connect to the iCloud contacts server p123-contacts.icloud.com (CardDAV). When it does, I get a “wrong site” certificate error. When I click “View certificate”, Thunderbird shows a valid certificate, but not for Apple/iCloud – it’s for login.kraftcom.de (issued by DigiCert / GeoTrust TLS RSA CA G1). I did not accept the exception.

From what I can see online, p123-contacts.icloud.com is a legitimate Apple domain, but it is also mentioned in some phishing / scareware contexts, which makes me even more unsure how to interpret this combination (Apple host + Kraftcom certificate). I’m also not sure whether this is simply a captive-portal / ISP login interception effect or something that should worry the wider community.

My questions:

Is this behavior expected in any scenario, or does it indicate a misconfiguration / MITM situation that Thunderbird should treat as a serious security issue?

Is there anything I should check or change on my side (Thunderbird config, network, ISP, certificates)?

Is there any additional diagnostic information I can provide (logs, screenshots) that would help you assess whether this is a Thunderbird issue or a network/ISP issue?

I specifically did not confirm the certificate exception and I would like to keep my setup secure, but I also want to make sure the community is aware if this is a broader problem.

Thanks in advance for any guidance

When someone sends me a msg, it displays along with the HTTP equivalent? Trying to figure out what setting is out of Wack such that I ses the HTTPS display of the message? Thanks

From Microsoft: "Recent Outlook Mobile updates introduced a stricter and more standards-aligned S/MIME message structure. The messages themselves are valid, but some third‑party mail clients such as Thunderbird do not fully support the way Outlook mobile structures the encryption. As a result, these clients may display a blank message body even though the encrypted content is present.

As a workaround, if you need to send encrypted mail to recipients who use third‑party clients, use Outlook desktop or Outlook on the web instead of Outlook mobile. These versions generate encrypted messages in a format that is more widely supported. "

I couldn't find this reported anywhere in these forums. Is there a solution being worked on to address this issue? Or has anyone found a workaround? We are stuck with using the Outlook app until the Thunderbird app supports S/MIME.

On Monday, 17 November 2025 the Security Certificate expired and I stopped receiving emails both on my computer and Andriod Device. As a result I am not receiving any emails through Thunderbird, but I can go to email host site to view them.

I logged in my computer tonight and clicked here and clicked there and the error message: "The certificate for mail.homebizmall.net is not valid for that server. Someone could be trying to impersonate the server. Create an exception but this could mean that someone has stolen your certificate and may be able to impersonate you."

So it seems the POP server and the SMTP server certificates were only valid for 3 months and I was FORCED to create the exception for both servers. Now I am worried the security has been breached.

By clicking here and clicking there, I stumbled on a few people who seem to have this problem, but each with their own limitations. My operating System is Windows 10 without the option to upgrade. I have not extended the security updates because I don't understand if it is just the operating system uploading to the cloud or everything on my computer and I am not prepared to do the latter.

Is it safe to leave the band aid on that I have used for the moment? Can I create an exception for the phone? Or is there a glitch that someone needs to walk me through fixing?

Thanking you in advance

Clinton

Hello.

I have added a new account to Thunderbird, and it is not syncing to show new messages. I am getting a pop-up from Tbird saying "The certificate for mail.newcreationloughborough.uk is not valid for that server ..." etc. I have gone through the procedure titled 'Troubleshooting email client warnings about invalid server certificates' to add Norton's certificate to my email app (as directed by the 'resolve' link in the 'Resolve email sync issues caused by certificate warnings' from the Norton EE Cyber Security add-on), and it tells me that the certificate is already installed.

What can I do next to get it to sync?

When I export my private key and add more user IDs to it, GPG adds extra feature flags that apparently aren't supported in Thunderbird. I've tried following the directions here, but GPG doesn't seem to do anything different with the --rfc4880 flag, and so importing the key back into Thunderbird still gets the "unsupported features" warning.

Even when creating a key in Sequoia-PGP, it seems to add an extra notation? And that also seems to make Thunderbird say it's an extra unsupported feature.

Am I doing something wrong?

Hi, I used to see the HTTPS designation in the Firefox address bar, but no longer see it lately. It does appear in the Edge address bar.

I do appreciate your help.

I am getting this message when I try to send email. All of my settings are correct. 'Sending of the message failed. Unable to communicate securely with peer: requested domain name does not match the server’s certificate.

Hello,

I had issue to verify the cert on android app for https://partners-enrichment.heytelecom.be. On Windows I didn't have the same issue.

version 144.0.2 / build id 20251027123126 / target arm64-v8a armeabi-v7a x86_64 Device: Samsung S22 / One UI 7.0 / Android version 15 / version S901U1UES8FYI2 / Security patch level September 1, 2025

Error: Secure Connection Failed, because its certificate issuer is unknown, the certificate is self-signed, or the server is not sending the correct intermediate certificates.

CertChain RCA: DigiCert Global Root G2 DCA: DigiCert Global G2 TLS RSA SHA256 2020 CA1 cert: partners-enrichment.heytelecom.be

1) I couldn't check the cert from the gui as on windows. Shield in search bar / Connection not secure doesn't opened the cert. Is it expected?

2) I found this helppage: https://support.mozilla.org/en-US/kb/setting-certificate-authorities-firefox

Unfortunately about:preferences isn't available on android (ref. https://www.reddit.com/r/firefox/comments/u593x0/how_to_access_to_aboutpreferences_on_android/)

I see in about:certificate which is the correct RCA (I verified the pem file with the root). DigiCert Global Root G2

Where do I check the Intermediate CA's (DCA)?

3) When exporting the RCA it has been download as: digicert-global-root-g2.pem.txt Why the txt at the end?

On Windows it downloads as digicert-global-root-g2.pem

Kind regards,

My mobile package site shows mixed-content warnings on Firefox — how can I fix this? Hello everyone, I run a website, which provides information about mobile phone prices and SIM packages in Bangladesh. Recently, I noticed that when users visit some pages in Firefox, the browser shows a “mixed-content” warning or blocks certain scripts. The same pages load fine in Chrome. All my URLs use HTTPS, and I’m using a LiteSpeed server with Cloudflare CDN. Could this issue be related to how Firefox handles external resources (like embedded operator banners or analytics scripts)? What’s the best way to debug and fix mixed-content problems in Firefox Developer Tools? Any detailed guidance or best practices would be greatly appreciated. Thanks in advance!

I am using Firefox version 140.4.0esr (64bit) Only on Firefox: when trying to connect with Pirates' web page, I have a block which reads:

Security Connection Failed - no common encription algorithems      Error Code: ssl_error_no_cypher overlap

To The Official Site of The Pittsburgh Pirates/pirates.com:Homepage

I have tried to contact them with no luck.

Can you help?

Thanks,

David Martin ([edited email from public community support forum])

I have just installed Thunderbird, am continually getting "certificate for outlook.office365.com" not trusted. I have added Thunderbird to my Malwarebytes exception list, but the the notice continue to come. I have ensured that OAuth2 is the correct authentication method, but the messages persist.

Is there something else I can do to stop these messages?

Would be very grateful, thank you.

Tim Harrison

Hello, I put the HTTPS mode Only mode on my OS for Imac, and I would like to put it on my iPhone 14, but I can't find how to do it, can you help me? THANKS. Best regards, christophe

Hi All. New to Thunderbird but liking what I see. Am trying to install a new certificate but keep seeing the same error message when I attempt to send an email:

Sending of the message failed. You specified that this message should be digitally signed, but the application either failed to find the signing certificate specified in your Mail & Newsgroup Account Settings, or the certificate has expired.

So here is what we know:

The certificate seems to work (I've sent test emails to a couple of test email accounts) when I use it in Outlook safe mode. Can't get it to work in normal Outlook mode because of add-ins, hence my switch to Thunderbird. They seem to appear fine in the S/MIME section for digital signing and encryption and I can even see the certificate when I go into Manage S/MIME Certificates.

Has anyone come across this before / got any ideas about how to resolve this? Thanks!!

I downloads Firefox, Firefox Nightly, Firefox Developer.

All edition not write SSLKEYLOG to file.

I set the SSLKEYLOGFILE env.

How to enable ssl key log file?