My organization uses S/MIME certificates to digitally sign and encrypt emails. We would like to add a disclaimer at the bottom of our emails, which can be done via the mail flow settings in Office 365.
However, if the message is edited by the server after it has been signed by the sender, the certificate will show as invalid.
Is there a way to add an organizational email footer without affecting S/MIME functionality? Or is the only solution to add the disclaimer to to the client's signature?