Can I use a Let's Encrypt Cert for Office365/Exchange


I am totally new to this whole area.

I would like to know if I can use a Let’s Encrypt certificate to sign email using Office356? Also, can I use it on my Office client?

If I can, can anyone tell me how to?




In short, NO.

If you are trying to have a certificate to “certify” or “sign (as identify)” yourself, you can’t.

First of all, Let’s Encrypt certificates are used to authenticate connections to a server (to prove that the server is the server), it would not be able to prove if a sender is a sender (person). Because it only contains the domain ( not the email addresses ( (in plain ways).

Also, when you talk about email certificate, you probably are talking about S/MIME certificate, which would include client-auth and digital signature, where Let’s Encrypt would only have server-auth and client-auth…

Thank you


(If you need a certificate for a TLS service provided by your mail server (like SMTPS, STARTTLS, or IMAPS), then Let’s Encrypt could provide that—but not for S/MIME.)


Thank you for the clarifications.

closed #5

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