On succesfull forward, set IMAP FORWARDED flag/keyword.
Found things about that in:
https://tools.ietf.org/html/rfc5550#section-5.9
https://tools.ietf.org/html/rfc5788#section-3.4.2
Thunderbird, K9 and apparently PHP IMAP “imap_setflag_full” does this too.
This is a show stopper here…