How do I disable S/MIME certificate signing in Apple Mail?
The Apple Mail program supports a rarely-used feature called S/MIME signing. This feature uses a “public key encryption certificate” to prove that a message you send really came from you. You can buy these certificates online (from companies unrelated to us).
If you’ve enabled that feature in the past, but the certificate has expired, people you send email to may see an error saying that their mail program is unable to verify your signature. If that happens, and you don’t want to obtain a new certificate from the service you got the original certificate from, you should turn off S/MIME signing in the Apple Mail program to stop other people from seeing the error when you send them mail.
To do this, look for the blue checkmark to the right of the “Subject” when you compose a new message (circled in green below):
The blue checkmark indicates that Apple Mail will try to sign it. To prevent that, click the blue checkmark so that it changes to a grey X, like this:
Apple Mail will then no longer try to sign your messages.
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