envelope.go
func (e *Envelope) GetHeaderValues(name string) []string {
	if e.header == nil {
		return []string{}
	}

	rawValues := (*e.header)[textproto.CanonicalMIMEHeaderKey(name)]
	var values []string
	for _, v := range rawValues {
		values = append(values, decodeHeader(v))
	}
	return values
}
func (e *Envelope) GetHeaderValues(name string) []string {
	if e.header == nil {
		return []string{}
	}

	rawValues := (*e.header)[textproto.CanonicalMIMEHeaderKey(name)]
	var values []string
	for _, v := range rawValues {
		values = append(values, decodeHeader(v))
	}
	return values
}
// GetHeaderValues processes the specified header for RFC 2047 encoded words and returns all existing
// values as a list of UTF-8 strings