builder.go
func (p MailBuilder) BCCAddrs(bcc []mail.Address) MailBuilder {
	p.bcc = bcc
	return p
}
func (p MailBuilder) BCCAddrs(bcc []mail.Address) MailBuilder {
	p.bcc = bcc
	return p
}
// BCCAddrs returns a copy of MailBuilder with the specified as the blind CC list.  This method only
// has an effect if the Send method is used to transmit the message, there is no effect on the parts
// returned by Build().