OTP SMS Best Practices: Improve Delivery and Stop Bounce Rates

An OTP is transactional: the user is waiting for it, the window is short, and a single failed delivery means a lost signup or a support ticket. Here is how to structure OTP delivery so it reliably gets through.

1. Use priority routing

OTPs should not queue behind marketing traffic. Choose a platform that lets you mark a message as priority so it goes out on the fastest route immediately. On Aion Messaging this is the priority: true flag on the send endpoint.

2. Keep messages short and clean

Six digits beats eight when speed matters. Stick to GSM characters: plain numbers and letters. Emojis or special characters push the message onto the 70-character Unicode rate and can trip content filters.

3. Set a tight expiry and resend policy

• Expire codes after 5 minutes or fewer.
• Allow a resend but rate-limit it (e.g. max 3 in 10 minutes).
• Reject repeated attempts after a small number of failures to stop brute force.

4. Track delivery, not just sends

Watch the delivery status on every OTP. If a destination shows a rising failed or "undelivered" share, that is usually a routing problem your provider should fix — or a sign you need a fallback route. Ask about automatic fallback to a second carrier.

5. Offer an SMS-free fallback

Some users are in areas with patchy coverage, or their carriers filter short messages. Offer a voice call or email fallback so verification never dead-ends. Platforms that support OTP verify (server-side code checking) make this safer: never return the code to your frontend.

6. Sender ID consistency

Use the same sender ID every time. Carriers and spam filters are suspicious when a "brand" appears from different alphanumeric IDs. Register a branded sender ID for production and keep your sender name stable.

Ready to implement? See the SMS API reference or try a free account with trial credits.