2.1.3.1.1.2 Bcc Recipients

To generate a Bcc header, clients add a recipient (2) to the Message object and set the value of the PidTagRecipientType property ([MS-OXOMSG] section 2.2.3.1) for that recipient (2) to 0x00000003.

When generating a message for outbound submission to SMTP, MIME writers MUST NOT copy Bcc recipients to the MIME Bcc header except for meeting and task messages. This also applies to the MIME Bcc header of attached messages. MIME writers MUST NOT copy Bcc recipients to the TNEF attRecipTable attribute, as specified in [MS-OXTNEF] section 2.1.3.3.22, for attached messages.

When generating a message for protocols such as POP3 and IMAP4, MIME writers SHOULD<10> copy Bcc recipients to the MIME Bcc header. This also applies to the MIME Bcc header of the attached messages. MIME writers SHOULD<11> copy Bcc recipients to the TNEF recipient table for attached messages.