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When I'm sending S/MIME mails from Microsoft Outlook, it attaches the full certificate chain to the out-going mail so that recipients can verify the signing key (similar to how TLS works). However, when I'm sending S/MIME from Mail.app, no certificate is attached. The behavior is similar to this discussion thread, but the thread is about iOS, while I care more about macOS.

  • There is very few questions under both smime and apple-mail; none of them is related to this question. – Franklin Yu Aug 28 '18 at 13:48
  • I think we’re going to need the details of which certs you’ve got installed where, what trust settings you have for them, and possibly what their key usage rules are. For example, does Mail.app even have access to all the relevant root and intermediate CA certs? Are they already installed and trusted in your keychain? – Spiff Aug 28 '18 at 15:51
  • @Spiff AFAIK if the key is installed incorrectly I wouldn't be able to send the S/MIME at all, right? Or is it expected to see the full chain in terms of Mail.app? One more proof: after I sent the mail, I saw the mail has a "signed" mark. I can double-click the mark to see the certificate chain. Anyway I would provide these details tonight, and attach it to the question. – Franklin Yu Aug 28 '18 at 17:05
  • @Spiff I got my certificate from Comodo, and they have an official guide. That's where I put my certificate. – Franklin Yu Aug 28 '18 at 17:20

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