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If Mail on your Mac can’t verify a server’s certificate
If a Mail server uses an SSL certificate that’s signed by an unknown authority (such as a self-signed certificate), the Mail app on your Mac displays a message indicating that it can’t verify the identity of the mail server. Although you can connect to the mail server, Mail will continue to warn you about it each time you open Mail. To stop the message from appearing, indicate that you trust the mail server’s certificate.