What Is SMS Delivery Rate?
Your SMS delivery rate is the percentage of submitted messages that receive a DELIVRD (delivered) DLR status. A 95% delivery rate means 5% of messages failed — which at 100,000 messages/month, is 5,000 undelivered messages.
Understanding why messages fail — and fixing the root causes — is one of the highest-ROI activities for any SMS programme.
Common Reasons for Delivery Failure
1. Invalid or Non-Existent Numbers (UNDELIV / err:001)
The most common cause. Numbers that have been:
- •Ported to a different carrier and not yet propagated
- •Deactivated by the network
- •Never allocated
- •Formatted incorrectly (missing country code, wrong length)
Fix: Run an HLR (Home Location Register) lookup before sending. HLR checks a number's network registration status in real time. Remove all unreachable numbers before sending.
2. Number Porting (UNDELIV / err:029)
When a subscriber moves from one network to another, routing databases take 24–72 hours to update. During this window, messages may be routed to the wrong network.
Fix: Use a gateway with up-to-date HLR and real-time number portability data. BulkSMSRates updates portability databases every 15 minutes.
3. Handset Off or Out of Coverage (UNDELIV / err:027)
The subscriber's phone is switched off, in flight mode, or out of network coverage. Messages are queued for delivery when the handset reconnects, up to the validity period (default: 48 hours).
Fix: Set an appropriate message validity period via API. For time-sensitive messages (OTPs, flash sales), use a short validity (30 minutes). For non-urgent messages, use 24–48 hours.
4. SMS Barred (UNDELIV / err:032)
The subscriber has SMS services disabled on their account (intentionally or due to arrears with their carrier).
Fix: Remove these numbers after 2–3 consecutive failures. They are unlikely to be deliverable.
5. Content Filtering / Spam Rejection (REJECTD / err:099)
Carriers and SMSCs filter messages containing:
- •URLs from flagged domains
- •Common spam phrases (free, winner, prize, click here)
- •Mismatched sender IDs
- •Messages without opt-out text
Fix: Use compliant message templates. Avoid spam trigger words. Register your sender ID. Include STOP keyword. Use a reputable short URL service.
6. Grey Route / Low-Quality Routing
Some providers use "grey routes" — unofficial, unmonitored paths through international carrier networks. These have low delivery rates, no genuine DLRs, and risk of sudden disconnection.
Fix: Use a provider with direct carrier connections. BulkSMSRates uses only direct and tier-1 routes with genuine DLR from destination networks.
How to Improve Your Delivery Rate
- 1.Clean your number list — remove numbers that have been invalid for 3+ months
- 2.Run HLR lookup before bulk campaigns
- 3.Check your sender ID — registered, recognisable sender IDs have higher delivery rates
- 4.Review your content — remove spam trigger words, ensure opt-out is present
- 5.Monitor DLR patterns — if UNDELIV rate increases suddenly, check for content filter changes
- 6.Segment by last delivery — exclude numbers with 3+ consecutive UNDELIV
Following these steps typically improves delivery rates from 85–90% to 95–99%+ for UK campaigns.