Claim an error only where the state failed, and freeze the two state lists

src/dlr.ts

@@ -131,10 +131,10 @@ export function parseReceipt(message: string): Receipt {
 type MessageType = 'intermediate' | 'other' | 'receipt' | 'unmarked';
 
 /** SMPP 3.4 Appendix B lists every other receipt state as final. */
-export const transientStates: MessageState[] = ['ENROUTE', 'SCHEDULED'];
+export const transientStates: readonly MessageState[] = ['ENROUTE', 'SCHEDULED'];
 
 /** Written by the far-end SME, not by the MC reporting on a message we submitted. */
-const smeMessageTypes: number[] = [
+const smeMessageTypes: readonly number[] = [
 	consts.ESM_CLASS.CONVERSATION_ABORT,
 	consts.ESM_CLASS.DELIVERY_ACKNOWLEDGEMENT,
 	consts.ESM_CLASS.USER_ACKNOWLEDGEMENT,

src/sms.ts

@@ -128,6 +128,7 @@ async function sendResp(
 /** The receipt as text, which is all of it a peer below SMPP 3.4 is allowed to be sent. */
 function receiptText(sms: Sms, smsId: string, status: MessageState): string {
 	const delivered = status === 'DELIVERED';
+	const failed = !delivered && !transientStates.includes(status);
 
 	return [
 		`id:${smsId}`,
@@ -136,7 +137,7 @@ function receiptText(sms: Sms, smsId: string, status: MessageState): string {
 		`submit date:${smppDate(sms.submitTime)}`,
 		`done date:${smppDate(new Date())}`,
 		`stat:${receiptCodes[status]}`,
-		`err:${delivered ? '000' : '001'}`,
+		`err:${failed ? '001' : '000'}`,
 		'text:',
 	].join(' ');
 }