That's implementation dependent! RFC822 (I assume it is outdated,
nevertheless) states:
4.5.3. BCC / RESENT-BCC
This field contains the identity of additional recipients of
the message. The contents of this field are not included in
copies of the message sent to the primary and secondary reci-
pients. Some systems may choose to include the text of the
"Bcc" field only in the author(s)'s copy, while others may
also include it in the text sent to all those indicated in the
"Bcc" list.
VSoup (& Souper I suppose) are transmitting an identical version of a
message to all recipients, because otherwise it has to send the
message twice with different content to different recipients (the
header is content!).
For VSoup this will perhaps change in the future!
-- Hardy Griech, Kurt-Schumacher-Str. 25/1, D-72762 Reutlingen