Function ComposeMessage

internal/outbound/compose.go:39–71

NOTE: Message-ID is intentionally omitted from composed messages. SES assigns its own Message-ID on send, and we capture it from the SMTP response. This avoids a mismatch where recipients see the SES ID but we tracked a different one. ComposeMessage builds an RFC 2822 email message (single content t

(from string, to []string, cc []string, subject, body, contentType, replyToMsgID string, references []string, fromDomain, replyTo, conversationID string)

// (Gmail) can't anchor the reply to the existing thread and forks a new one.
// With the full chain, the client matches on ANY prior ID and threads correctly.
func ComposeMessage(from string, to []string, cc []string, subject, body, contentType, replyToMsgID string, references []string, fromDomain, replyTo, conversationID string) ([]byte, error) {
	if contentType == "" {
		contentType = "text/plain"
	}

	var buf strings.Builder
	writeHeader := headerWriter(&buf)

	writeHeader("From", from)
	if len(to) > 0 {
		writeHeader("To", strings.Join(to, ", "))
	}
	if len(cc) > 0 {
		writeHeader("Cc", strings.Join(cc, ", "))
	}
	if replyTo != "" {
		writeHeader("Reply-To", replyTo)
	}
	writeHeader("Subject", mime.QEncoding.Encode("utf-8", subject))
	writeHeader("Date", time.Now().UTC().Format(time.RFC1123Z))
	writeHeader("MIME-Version", "1.0")
	writeHeader("Content-Type", contentType+"; charset=utf-8")

	writeThreadingHeaders(writeHeader, replyToMsgID, references)
	if conversationID != "" {
		writeHeader("X-E2A-Conversation-ID", conversationID)
	}

	buf.WriteString("\r\n")
	buf.WriteString(body)

	return []byte(buf.String()), nil
}

// ComposeMultipartMessage builds an RFC 2822 multipart/alternative email with text and HTML parts.
// If htmlBody is empty, falls back to a single text/plain message via ComposeMessage.