If the SSL certificate which is installed on the server does not match with the mail domain which you have configured on the Thunderbird Email Client application and you send emails from it, you will get an SSL certificate mismatch error.
With the warning message, Thunderbird tries to tell us that there is a problem with the SSL certificate. However, since you decided to take the risk anyway, Thunderbird should establish a secure connection and not show any warning message unless it gets a different mismatch certificate later on.
If you add a permanent security exception and are prompted to add the same exception over and over, your certificate store may be corrupted.
Due to SSL certificate conflict errors, the following are possible reasons for not receiving Thunderbird emails.
1) Domain Name Mismatch
2) SSL Certificate Expired
3) Issuer's Certificate Unknown
4) Certificate Revoked
1) Domain name mismatch
-> This error occurs when you connect securely to a server whose domain does not match the domain name in the returned certificate. This means that the website/server you are using may not be what you want. Also, you may change your domain but not purchase a certificate for the new domain.
-> It is also possible that the system administrator installed the SSL certificate incorrectly when it expired.
Solution: To fix this error, you need to ensure that the domain name you entered in the Incoming server's hostname and Outgoing server hostname field has a valid SSL Certificate.
You can check the status of the SSL Certificate from the below URL:
https://www.sslshopper.com/ssl-certificate-tools.html
2) SSL Certificate Expired
-> If your email domain's SSL certificate has expired and you do not renew it in time, you will receive an SSL warning message.
-> You will also see a warning message regarding SSL certificates if the SSL certificate expiration date is later than the system date. This may be due to the system having the wrong time, moving to another time zone, or the certificate being too old (expired).
Solution: To fix this error, you need to renew the SSL Certificate if it is expired. For that, you need to contact your hosting service provider where your website is hosted. If your system has the wrong date and time, you need to set the correct one to fix the issue.
3) Issuer's Certificate Unknown
-> If you receive an error message stating that the certificate is not trusted because the Issuer's certificate is unknown or the website is certified by an unknown authority.
-> This issue can also occur if your site has a self-signed certificate
-> Click the View Certificate button to view the certificate and see the organization listed under Issuer to see who the CA is.
Solution: To check the exact error, you can use online SSL Certificate tester tools such as SSL Certificate Tester.
-> A possible cause for this error is that the server has a self-signed certificate installed. Unfortunately, browsers do not trust self-signed certificates because they are generated by the server, not by a certificate authority.
Suppose you get a self-signed certificate on your server after installing a paid certificate. In that case, we recommend that you should review the SSL Certificate installation instructions and make sure you follow all instructions.
If you have completed all the installation steps and are still getting the same error, you will need to generate a new CSR for the domain from the server and re-issue the certificate.
-> Another common cause of this error Certificate Unknown is that the certificate is not correctly installed on the server (or servers) where the domain is hosted. You can check more on the error using an online SSL certificate tester tool. An incomplete installation shows certificate files and broken red chains in the tester tools.
To resolve this issue, install an intermediate certificate file on the server where your domain is hosted.
After installing the intermediate certificate, validate the installation again with the SSL Certificate Tester. In the tester, an incomplete installation shows multiple certificate files connected by an unbroken blue chain.
4) Certificate Revoked
If you get a certificate is revoked error message, it means that the certificate is invalid and cannot be used. Older versions of Thunderbird did not check for revoked certificates.
Solution: To fix this error, you need to ensure that the certificate installed on the mail domain is not canceled, expired, or removed. You need to reissue it to fix the error if it is removed or canceled.
