You should start from here: https://learn.microsoft.com/en-us/exchange/security-and-compliance/smime-exo/configure-smime-exo
How to digitally sign an email
We have M365 subscription. To enhance mail security, we are planning to add a digital signature to mail. So anyone would send email, it will be digitally signed using user or computer certificate to claim the ID of sender that is from a genuine user, not an imposter. How can we do it? appreciate your help.
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Aaron Xue-MSFT 2,586 Reputation points Microsoft Vendor
2022-01-31T01:45:27.103+00:00 Hi @Mohd Arif ,
Agree with crypt32.
You need to install the S/MIME before you sign the message.
I have done some research, here’s an article for you reference.
https://www.howtogeek.com/665438/how-to-digitally-sign-or-encrypt-emails-in-outlook-online/Please Note: Since the web sites are not hosted by Microsoft, the links may change without notice. Microsoft does not guarantee the accuracy of this information.
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Limitless Technology 39,811 Reputation points
2022-01-31T09:47:27.097+00:00 Hi there,
-On the File tab, click Options >Trust Center.
-Under Microsoft Outlook Trust Center, click Trust Center Settings.
-On the Email Security tab, under Encrypted Mail, select the Add digital signature to outgoing messages check box.Here is a link for a detailed description of the process that you must follow.
Secure messages by using a digital signature
https://support.microsoft.com/en-us/office/secure-messages-by-using-a-digital-signature-549ca2f1-a68f-4366-85fa-b3f4b5856fc6
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