AddHeaderField
status_t AddHeaderField(uint32 encoding,
status_t AddHeaderField(const char* field_name,
mail.AddHeaderField(B_MAIL_TO, to.String());
mail.AddHeaderField(B_MAIL_CC, cc);
mail.AddHeaderField(B_MAIL_BCC, bcc);
mail.AddHeaderField(B_MAIL_SUBJECT, subject);