kindly find below a patch which I have been using in my local setup for
about a year without seeing any hiccups. So I thought it would be worth
proposing to the developers.

The issue I saw with S/MIME encrypted messages was, that the processing
of the MIME tree stopped after decryption. It turned out there was no
handler for application/(x-)?pkcs-mime type entities in notmuch-show.el,
and the "handler of last resort" notmuch-show-insert-part-*/* was
called. Thus, this patch just adds a notmuch-show handler for
application/(x-)?pkcs-mime type entities.

RFC 8551 defines that there is a single child element only ("A body can
consist of a single MIME entity or a tree of MIME entities (rooted with
a multipart).", clause 3.1), hence no list processing of the children is
needed (there's only one).

Hoping to have helped, and looking forward to your thoughts,


>From d10f4964173ae644077de91b56a98d48f7253ce8 Mon Sep 17 00:00:00 2001
From: Alexander Adolf <alexander.ad...@condition-alpha.com>
Date: Fri, 25 Mar 2022 14:13:28 +0100
Subject: [PATCH] emacs/smime: render decrypted MIME entities in notmuch-show

When processing encrypted S/MIME messages, after decryption the "last
resort" handler notmuch-show-insert-part-*/* was called, because there
was no application/pkcs7-mime handler, resulting in the decrypted
contents not being displayed.

This commit adds a new function
notmuch-show-insert-part-application/pkcs7-mime (and an alias
notmuch-show-insert-part-application/x-pkcs7-mime for the legacy MIME
type) to render the S/MIME protected part after decryption.
 emacs/notmuch-show.el | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/emacs/notmuch-show.el b/emacs/notmuch-show.el
index 7c1f02c9..b7edfc98 100644
--- a/emacs/notmuch-show.el
+++ b/emacs/notmuch-show.el
@@ -712,6 +712,23 @@ will return nil if the CID is unknown or cannot be retrieved."
 (defun notmuch-show-insert-part-application/pgp-encrypted (_msg _part _content-type _nth _depth _button)
+(defun notmuch-show-insert-part-application/pkcs7-mime (msg part _content-type _nth depth _button)
+  "Render S/MIME protected content after decryption.
+An alias for this function is also defined to handle entities
+using the legacy application/x-pkcs7-mime MIME type."
+  (let* ((encstatus (car (plist-get part :encstatus)))
+	 (inner-part (car (plist-get part :content))))
+    ;; Insert a button detailing the encryption status.
+    (notmuch-crypto-insert-encstatus-button encstatus)
+    (if (not (string= (plist-get encstatus :status) "bad"))
+        ;; Show all decrypted parts.
+        (notmuch-show-insert-bodypart msg inner-part depth))))
+;; Support for the legacy "x-" type.
+(fset 'notmuch-show-insert-part-application/x-pkcs7-mime
+  'notmuch-show-insert-part-application/pkcs7-mime)
 (defun notmuch-show-insert-part-multipart/* (msg part _content-type _nth depth _button)
   (let ((inner-parts (plist-get part :content))
 	(start (point)))

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