Signing emails in Mail on macOS

First, you must manually set up a trust for the I.CA root certificate

You need to open the "Keychain", on the left side select "System". Then you will find the root certificate

I.CA Root CA/RSA, right-click on it, select Information and grant the certificate the trust as shown below:

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Then just launch the Mail app and start writing a new message. After that, the email is automatically signed. You will always see a pipe icon next to the signed message, like in the image below.

It is necessary that the email from which you want to send electronically signed messages matches the email that is listed in the certificate.

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