>> Subject: Test =?ISO-8859-1?Q?=C6=D8=C5_=E6=F8=E5?=
> No advice on my side, I don't even know the name of the encoding above :)
That's quoted-printable ("?Q?") for subjects, with encoding in ISO-8859-1.
GNATCOLL.Email.Util includes encoders and decoders for this format.