Support Gmail-style bcc header insertion (bcc recipients have their own email in bcc headers)

[6peterlu]

Is your feature request related to a problem? Please describe.

Gmail will add bcc recipients to their respective MIME headers, so that each bcc recipient sees their own bcc information in the headers but not others. Haraka appears to create identical MIME headers for each recipient.

Describe the solution you'd like

Haraka adds bcc headers for bcc recipients so that they can see themselves in the headers but not other bcc recipients.

Describe alternatives you've considered

An alternative is to compose the MIMEs outside of Haraka and send multiple SMTP requests, one for each bcc recipient, but this seems like functionality that is ideally contained within haraka.