If I am using PGP to encrypt mails whther it is send as a MIME (Multipurpose Internet Mail Extensions) message, and S/MIME and PGP are used for providing email security.

When I send PGP message using a email client to my another web based account, what I got was a an attachment.

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    Can you explain why you care? As an old messaging admin, I'd like nothing better than to read a detailed explanation of the interoperation of MIME and PGP, liberally quoting RFC 2015, but I suspect you may have a specific practical problem behind this that you want answering. – Graham Hill Mar 28 '12 at 10:41
  • @Graham Its just for academic point of view only. Was looking for differentiating the two. But though how they would act together when I employ PGP in my mail agent like thunderbird – user2315 Mar 28 '12 at 15:17
  • to enable inline support defaults write org.gpgtools.gpgmail UseOpenPGPInlineToSend -bool YES – davidcondrey May 12 '16 at 6:40

There are two ways of signing/encrypting email with PGP: Inline-PGP and PGP/MIME.

Inline PGP takes a plain text message, and signs it or encrypts it, and the signed or encrypted message is sent as a single plain text message.

PGP/MIME uses a MIME-type of either multipart/signed (or for encrypted mails multipart/encrypted.) The original unsigned message is left alone, and an additional MIME part is added which contains the signature. This would MIME part would be seen by any non-PGP-aware MUA as being just some random attachment.

The difference between PGP and S/MIME is more in the formatting of the certificates, and the trust model used. PGP uses a web-of-trust, and S/MIME uses a PKI based on Certification Authorities. A PGP public key file is formatted very differently from an X.509 certificate.

In terms of mixing PGP and S/MIME, its possible to do, if you use inline-PGP, and then S/MIME sign the message. (Why one would want to is another question.)

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MIME convert raw-binary data to the form, which can be transferred by e-mail.

PGP|S/MIME encrypt data, produced by MIME

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