I suppose one could define an SMTP extension that basicly would
say that the SMTP server will remove the BCC line before the message
is sent to the end user (or server that doesn't handle this extension).
This would only be a problem with several BCC-recipients behind the
same SMTP server (or MX host). If you're sending one message to one
To-recipient and two Bcc-recipients at different hosts, you're already
sending three copies of the same message.
If you're concerned about network load, look into routing tables
and make intelligent guesses when several recipients could share
one copy of the message the first part of the network route.
For example I'm sending this message to one address at ibm.net and
one at Colorado.EDU from abc.se. I suppose two copies of the message
will be sent across the atlantic, when message would have been enough.
Another option is to compress messages before they're transferred.
I don't think removing the Bcc-line completely so you can send only
one copy of the message outweights the confusion it (probably) creates.
Christer Romson