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Symptom

Until recently, you could easily retrieve and send emails. But suddenly the error message "Server identity can not be verified" appears.

Cause

The problem occurs only on Apple devices, especially iPhone. This is not a server-side issue, but a software issue on the Apple device. The server certificate has been extended correctly and works with all other systems problem-free, e.g. Thunderbird, Outlook, Webmail, as well as in Internet-related transactions with other mail servers. Although Let's Encrypt has been accredited for years and millions of times as a trust center, your iOS mail does not contain the necessary root certificate to automatically validate Let's Encrypt SSL certificates. The Apple software therefore considers the extended server certificate to be an untrusted, e.g. self-signed certificate.

Solution

According to numerous forum discussions in Apple forums, the problem can be solved on the devices only by deleting and re-installing the mail account. The reason for this is that the previously existing "Continue" switch was removed in the error message by Apple and the users of this software therefore no longer have the opportunity to inform the device that the certificate should be accepted. Further information can be found in the following sources.

Probably the right technical solution would be to update the software so that it knows all current SSL/TLS root certificates. You may be able to do this by updating your iOS Mail software or operating system. For further questions, please contact Apple support.

Overview of frequently asked questions