I have tried to implement S/MIME, however, I've found out that S/MIME seems to be obsolete, in my perspective. Since S/MIME cannot operate on Google Chrome or Edge.
Another solution I found is Gpg4win but it also doesn't work with Outlook 365.
Is there any other solution I could implement so that I could sign email from both Outlook client and Outlook 365?
Those tools such as AdobeSign, DocuSign, SigningHub works only for signing documents but not sign email.