Address list group syntax, as described in RFC 5322 (https://tools.ietf.org/html/rfc5322#section-3.4) is of the form "group1: adress1, address2;". It is seldom used, except to indicate undisclosed recipients, i.e., as "undisclosed-recipients:;". Its usefulness in this latter case is to omit addresses in the header. This use may be expanded in the future, see RFC 6854 (https://tools.ietf.org/html/rfc6854).

IMAP servers deal with group syntax in the envelope as follows (RFC 3501 Page 76, https://tools.ietf.org/html/rfc3501#section-7.4.2):

“[RFC-2822] group syntax is indicated by a special form of
address structure in which the host name field is NIL.  If the
mailbox name field is also NIL, this is an end of group marker
(semi-colon in RFC 822 syntax).  If the mailbox name field is
non-NIL, this is a start of group marker, and the mailbox name
field holds the group name phrase.”

For example "group: a@b, C <c@d>;" becomes

    (NIL NIL group NIL) (NIL NIL a b) (C NIL c d) (NIL NIL NIL NIL)

When faced with a message with such group syntax in an address list, Trojita outputs

    group@, a@b, C <c@d>, @

in the GUI Envelope view.

When replying to such a message,

    group@[]
    ""@[]

appear in the reply list.

Based on this, my impression is that both the underlying model and the GUI do not deal with group syntax correctly.

A thorough fix would be to fix the underlying model and GUI parts. A simple fix could perhaps consist of filtering out invalid addresses such as 'group@' and '@' from the address list. An intermediate fix would be to filter them out when replying and dealing correctly with group syntax when constructing the envelope view.

Reproducible: Always