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How to fix the errorError downloading email from mail server at outlook.office365.com:993 - An error occurred while attempting to establish an SSL or TLS connection. The server's SSL certificate could not be validated for the following reasons: • The ser

Mohamed 20 Reputation points
2023-10-12T10:40:35.2766667+00:00 
Error downloading email from mail server at outlook.office365.com:993 - An error occurred while attempting to establish an SSL or TLS connection.

The server's SSL certificate could not be validated for the following reasons:
• The server certificate has the following errors:
  • The revocation function was unable to check revocation for the certificate.

  • The revocation function was unable to check revocation because the revocation server was offline.

• An intermediate certificate has the following errors:
  • The revocation function was unable to check revocation for the certificate.

  • The revocation function was unable to check revocation because the revocation server was offline.

 --> The remote certificate is invalid according to the validation procedure.
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Accepted answer
  1. Iohann Gessenay 161 Reputation points
    2023-10-18T20:36:10.6533333+00:00

    Network Issues: Ensure that your network is not blocking access to the revocation servers. Some firewalls or network configurations might block access to CRL or OCSP servers.

    Time & Date: Ensure that the system's date and time are correct. An incorrect system time can cause issues with certificate validation.

    Temporary Revocation Server Downtime: It's possible that the revocation server was temporarily down. You can try connecting again after some time.

    Certificate Pinning: If you trust the server and know that the certificate is valid, you could consider certificate pinning, where you hardcode the certificate in the client. However, this is generally not recommended unless you're sure about the server's authenticity and have a mechanism in place for updating the pinned certificate.

    Bypass Revocation Check: Most email clients or applications provide an option to bypass the certificate revocation check. While this can help you establish a connection, it's a less secure method. If you choose this route, ensure that it's only a temporary measure and that you revert to the secure settings once the original issue is resolved.

    Update Email Client: Ensure that your email client or application is up-to-date. Sometimes, updating the client resolves certificate-related issues.

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