SMS is not delivering
A2P 10DLC, opt-out keywords, and carrier filtering.
4 min read Updated 2026-04-24
- US: A2P 10DLC not registered — number blocked or massively throttled.
- Recipient replied STOP — they are in DND for SMS.
- Message contains a blocked URL shortener — use your own tracking domain.
- Recipient number invalid or landline — invalid numbers show "undelivered" in the log.
- Sending rate too fast — throttle to 1-3 msgs/sec/number.
- Toll-free number not verified — unverified TFNs are heavily throttled.