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