Twilio error 21610 means the destination number has previously replied STOP to your Twilio number or short code, and Twilio's opt-out management system is blocking the message before it is submitted to any carrier. This is not just a technical error: sending to an opted-out number violates the Telephone Consumer Protection Act (TCPA) in the US and equivalent regulations in other markets. The correct response to 21610 is to permanently remove the number from your active send list, not to retry or work around the block.
What Causes This Error
Twilio automatically tracks STOP, STOPALL, UNSUBSCRIBE, CANCEL, END, and QUIT replies sent to your Twilio numbers and short codes, and records each opt-out in its opt-out database keyed to the specific number pair (the recipient number plus your From number). When you attempt to send a message from the same From number to a recipient who has opted out, Twilio returns 21610 before the message reaches any carrier. A common operational cause is an application that reads contact lists from a database without first checking against Twilio's opt-out list, particularly after a database migration or import that brought in contacts without their opt-out status. Using a different From number to re-send to an opted-out number does not trigger 21610 for the new number pair, but it does violate the spirit of the opt-out request and exposes your business to legal liability.
How to Fix It Step by Step
Remove the opted-out number from your active sending list immediately in your application database by marking it as unsubscribed with the timestamp of the opt-out. To view opt-outs for a specific number or short code, navigate to the Twilio Console, go to Phone Numbers, then Manage, then Active Numbers, click on the From number, and scroll to the Messaging section where you will find a link to view opt-outs associated with that number. If a contact has opted out and later wants to re-subscribe, they must send START, YES, or UNSTOP to your From number, at which point Twilio automatically removes them from the opt-out list and subsequent API calls to that number pair will succeed again. Never attempt to clear opt-outs programmatically using the Twilio opt-out deletion API without first confirming that the subscriber has explicitly re-opted in via a START message, as arbitrary opt-out removal violates TCPA and can result in significant legal penalties.
How to Prevent It from Recurring
Implement a real-time opt-out synchronization system that listens to inbound messages on your Twilio numbers via the inbound message webhook, identifies STOP-keyword messages, and immediately marks the corresponding contact as unsubscribed in your application database before the next send cycle begins. Before every bulk send campaign, query Twilio's opt-out list for each From number using the Twilio REST API at /2010-04-01/Accounts/{AccountSid}/Messages endpoint or the IncomingPhoneNumbers API to retrieve current opt-out records, and subtract any opted-out numbers from your send list. Add a per-message pre-send check in your send function that queries your local opt-out table before calling Messages.create, so that even if the synchronization job has a lag, a local suppression check provides a second layer of protection. Monitor your 21610 error rate in your StatusCallback logs and treat any occurrence as a data quality issue: each 21610 represents a contact that was in your send list despite being opted out, and each one needs to be traced back to understand how it bypassed your suppression logic.
When to Call a Specialist
If you are receiving 21610 errors at a high rate despite having an opt-out synchronization system in place, the synchronization logic has a race condition or gap that is allowing opted-out numbers to reach the send queue, and a specialist can audit the data flow to identify the specific gap. For businesses that manage messaging on behalf of multiple clients (ISVs, SaaS platforms), maintaining accurate opt-out lists per client and per sender number pair becomes complex and benefits from a specialist-designed architecture that handles opt-out propagation correctly across all tenants. You should also seek specialist help if you need to implement a legally compliant opt-back-in flow that re-subscribes users after they have sent STOP, ensuring the re-subscription flow satisfies TCPA requirements before you remove them from your suppression list. TCPA compliance failures carry statutory damages of $500 to $1,500 per message, making this an area where specialist guidance pays for itself immediately.
Conclusion
Error 21610 is a TCPA-protective hard block that must be respected: remove the number from your send list and maintain a robust opt-out management system going forward. If this error is blocking your production system, contact our team and we will diagnose and fix it within the hour.
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