When testing with Exchange I've found that the Bcc mail header is passed on to all recipients of the email. I'm not sure whether other SMTP servers have the same issue.
The RFCs are a bit murky on this issue, but I did find some support for this approach in RFC 2822:
In the first case, when a message
containing a "Bcc:" field is prepared to be sent, the "Bcc:" line is
removed even though all of the recipients (including those specified
in the "Bcc:" field) are sent a copy of the message
When testing with Exchange I've found that the
Bccmail header is passed on to all recipients of the email. I'm not sure whether other SMTP servers have the same issue.The RFCs are a bit murky on this issue, but I did find some support for this approach in RFC 2822: