builder.go
func (p MailBuilder) Header(name, value string) MailBuilder {
	// Copy existing header map
	h := textproto.MIMEHeader{}
	for k, v := range p.header {
		h[k] = v
	}
	h.Add(name, value)
	p.header = h
	return p
}
func (p MailBuilder) Header(name, value string) MailBuilder {
	// Copy existing header map
	h := textproto.MIMEHeader{}
	for k, v := range p.header {
		h[k] = v
	}
	h.Add(name, value)
	p.header = h
	return p
}
// Header returns a copy of MailBuilder with the specified value added to the named header.