Issue with parsing of forwarded messages with attachments and quotes in the subject

[Giovanni Salvatore Fois]

Hi all,
           at work we use Roundcube acting as a mail client for the  Dovecot Imap server.

In Roundube the messages are parsed through the Imap BODYSTRUCTURE command
If a message contains forwarded messages and attachments and some of the messages contains
a quote (") in the subject, then the resulting BODYSTRUCTURE appears to be malformed.
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After some digging through the code of Dovecot, it seems to me that the problem is related to the 
function: imap_append_string_for_humans
defined at the line 120 of the file src/lib-imap/imap-quote.c  (dovecot sources v. 2.2.29)

Said function is supposed to return a quoted version of the input string while removing unwanted
characters from the input data.

But,  if I call it with a string containing quotes as argument (eg: I am a lazy " programmer)  in
the resulting output the quotes aren't escaped and the output string is not quoted.
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This is related to the stated issue through the function calls in:
part_write_body   [ defined at line 142 of imap-bodystructure.c ]
which calls imap_envelope_write [defined at line 46 of imap-envelope.c]
which finally calls imap_append_string_for_humans
----

Thank you for your time.

Best regards,
Giovanni S. Fois