builder.go
func (p MailBuilder) BCC(name, addr string) MailBuilder {
	p.bcc = append(p.bcc, mail.Address{Name: name, Address: addr})
	return p
}
func (p MailBuilder) BCC(name, addr string) MailBuilder {
	p.bcc = append(p.bcc, mail.Address{Name: name, Address: addr})
	return p
}
// BCC returns a copy of MailBuilder with this name & address appended to the BCC list.  name may be
// empty.  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().