envelope.go
func (e *Envelope) GetHeader(name string) string {
	if e.header == nil {
		return ""
	}
	return decodeHeader(e.header.Get(name))
}
func (e *Envelope) GetHeader(name string) string {
	if e.header == nil {
		return ""
	}
	return decodeHeader(e.header.Get(name))
}
// GetHeader processes the specified header for RFC 2047 encoded words and returns the result as a
// UTF-8 string