S/MIME signed messages support in Gmail

From RFC5751 

"There are two formats for signed messages defined for S/MIME:

• application/pkcs7-mime with SignedData
• multipart/signed.

In general, the multipart/signed form is preferred for sending, and receiving agents MUST be able to handle both."

As per 1st of April, 2020 the situation with S/MIME signed messages support for free Gmail accounts is as following:

multipart/signed

• Gmail to Gmail: NOK  (since 2013)
• Gmail to External Mail: NOK (since 2013)
• External Mail to Gmail: OK 
• 
  

application/pkcs7-mime with SignedData

• Gmail to Gmail: NOK (since 2017)
• Gmail to External Mail: OK
• External Mail to Gmail: OK 

NOK means Gmail service mangles a message in transit by:

• repacking MIME entities and changing boundaries
• changing the content type of the message to multipart/mixed
• restricting access to S/MIME signature for multipart/signed messages

The history of the above points:

• 2013 - Google has started to mangle multipart/signed messages inside Gmail
• 2017 - Google has started to mangle application/pkcs7-mime with SignedData inside Gmail