webmail: parts of type message/rfc822 (and message/global) aren't displayed

[mjl-]

MIME messages can include full other messages, parts with types message/. The webmail is currently completely ignoring such nested parts in the webmail. In the case of a multipart/mixed message/part, we should have a way to show both the "regular" parts and message/ parts, possibly with each message-part only fully displayed in a popup. These message parts have headers that should be displayed too. Special care may be needed around DSNs, I think they could be truncated.

Reported by mteege, who saw multipart/mixed messages with a message/rfc822 and a text/plain on a mailing list (where the text/plain part was a mailing list footer added by the mailing list). (footer added by mailing list)