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