SUMMARY
***
For some time (maybe after I changed mail provider), kmail does not show certain encapsulated messages.
Same encapsulated mail can be seen on eg K9mail on Android (or in Webmail).
***

WHAT HAPPENS:

Encapsulated message shows no from, no date, no content.
Opening encapsulated message is no better. Using the V keyword in this empty encapsulated message window shows the base64 code.

EXPECTED RESULT:
I should see the encapsulated message.

DETAILS:
Looking at the main message source, I see this structure:

 [Some header informations...]
Content-Type: multipart/mixed;
 boundary="_=_swift_1699451300_54c0e6e25c483d4a00e2d82bbf0aae60_=_"
[some more header here ...]

--_=_swift_1699451300_54c0e6e25c483d4a00e2d82bbf0aae60_=_
Content-Type: multipart/alternative;
 boundary="_=_swift_1699451300_ef16521f950d446f2fe191e04a841197_=_"

--_=_swift_1699451300_ef16521f950d446f2fe191e04a841197_=_
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: quoted-printable

[plain text message here ...]

--_=_swift_1699451300_ef16521f950d446f2fe191e04a841197_=_
Content-Type: text/html; charset=utf-8
Content-Transfer-Encoding: quoted-printable

[html message here ...]

--_=_swift_1699451300_ef16521f950d446f2fe191e04a841197_=_--

--_=_swift_1699451300_54c0e6e25c483d4a00e2d82bbf0aae60_=_
Content-Type: message/rfc822;
 name=confirmation-de-votre-commande-du-mercredi-8-novembre-2023.eml
Content-Transfer-Encoding: base64
X-WS-Attachment-UUID: 9fef11c7-3b6f-495c-a36f-95ee114544b3
Content-Disposition: attachment;
 filename=confirmation-de-votre-commande-du-mercredi-8-novembre-2023.eml

[base64 encoded string here ...]

--_=_swift_1699451300_54c0e6e25c483d4a00e2d82bbf0aae60_=_--


SOFTWARE/OS VERSIONS
Linux/KDE Plasma: 
(available in About System)
KDE Plasma Version: 5.27.8
KDE Frameworks Version: 5.110.0
Qt Version: 5.15.10

ADDITIONAL INFORMATION