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copied!<p>Actually, the code works just fine. It creates the proper mail with all the right headers including bcc.</p> <p><strong>How does the mail client display bcc?</strong></p> <p>The <a href="http://en.wikipedia.org/wiki/E-mail_client" rel="nofollow noreferrer">mail client</a> (e.g. Python or MS Outlook via IMAP or MAPI in my case) decides whether and how to display bcc-headers. Outlook for example doesn't display bcc headers from an IMAP folder. This is a feature to hide bcc recipients from each other where they have not been stripped away from the mail before (it is not clear from the standard whether one bcc recipient is allowed to see all other bcc recipients or not, see <a href="http://en.wikipedia.org/wiki/Blind_carbon_copy#Visibility" rel="nofollow noreferrer">Wikipedia</a>).</p> <p><strong>Who handles bcc upon sending an email?</strong></p> <p>Suppose now that we have drafted a message in a mail client and stored it in an IMAP or MAPI folder. The server providing the IMAP / MAPI folders leaves the draft message unchanged. What happens to the bcc-headers upon sending the mail is implementation dependent, and might depend both on the mail client and the <a href="http://en.wikipedia.org/wiki/Mail_transfer_agent" rel="nofollow noreferrer">mail transfer agent</a> (e.g. MS Exchange Server in my case). In a nutshell, people do not agree whether the mail client or the mail transfer agent is reponsible for removing bcc headers. It seems however to be the case that a majority of developers is of the opinion that it is the mail client's business with the mail transfer agent not touching the mail (e.g. MS Exchange, MS SMTP, Exim, OpenWave). In this case, the mail transfer agent sends the email to the recipient as defined in the <code>RCPT TO:</code> of the <a href="http://en.wikipedia.org/wiki/SMTP" rel="nofollow noreferrer">SMTP</a> communication, and leaves the email unchanged otherwise. Some other mail transfer agents strip bcc headers from emails however (e.g. sendmail, Lotus Notes). A very thorough discussion can be found on the Exim mailing list starting <a href="http://lists.exim.org/lurker/message/20040722.043422.7a079b9f.hu.html#exim-dev" rel="nofollow noreferrer">here</a>.</p> <p>In the case of MS Outlook and MS Exchange, MS Outlook never sends bcc (but sends individual emails for each bcc recipient) and MS Exchange does not touch the email headers, but sends the full email (possibly including bcc recipients) to the recipients defined in <code>RCPT TO:</code>.</p> <p><strong>Conclusion</strong></p> <p>I did not understand that there is no guaranteed behavior for bcc, and that usually the client handles bcc. I will rewrite my Python code to loop over bcc recipients and generate one email for each bcc recipient.</p>

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