Function writeThreadingHeaders

internal/outbound/compose.go:237–246

writeThreadingHeaders writes the In-Reply-To and References headers per RFC 5322 § 3.6.4. References is the full ancestor chain in conversation order (oldest → newest) and must be space-separated message-ids; mail clients use it to anchor a reply to an existing thread by matching ANY id in the chain

(writeHeader func(string, string), replyToMsgID string, references []string)

// non-empty references slice for any reply that may reach a recipient who
// did not see the immediate parent (multi-party / agent-mediated threads).
func writeThreadingHeaders(writeHeader func(string, string), replyToMsgID string, references []string) {
	if replyToMsgID != "" {
		writeHeader("In-Reply-To", replyToMsgID)
	}
	if len(references) > 0 {
		writeHeader("References", strings.Join(references, " "))
	} else if replyToMsgID != "" {
		writeHeader("References", replyToMsgID)
	}
}

func headerWriter(buf *strings.Builder) func(string, string) {
	return func(key, value string) {