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.

  • gmail supports s/mime. paubox.com/blog/gmail-encryption-settings s/mime works at the email header level, so it's supported at the client level, and a browser isn't an email client. – Steve Sether Aug 29 '18 at 2:47
  • Thank you for suggestion but my company only use Exchange online :( – Kong Aug 29 '18 at 3:12
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    "The things is I have to use only Internet Explorer which no one in my company is using IE now. " - this comment does not make much sense for me since in your question you explicitly mention Chrome and Edge as the relevant browsers for you. Anyway, S/MIME is officially support for Office 365 and Exchange so I'm not sure what alternatives you expect and why. Also, S/MIME is not considered obsolete. – Steffen Ullrich Aug 29 '18 at 4:43
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    GPG4O should work in Outlook ánd O365. – user185436 Aug 29 '18 at 13:38
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    @SteffenUllrich I'm guessing that by "The things is I have to use only Internet Explorer" the OP means that solution only works (or, at least, they think it only works) when using IE... since most of the OPs users don't use IE (they use Chrome/Edge), then this is a problem. – TripeHound Aug 29 '18 at 13:55