Fix Twilio Error 21610 in Sparkforge SMS

By Sparkforge Support Team ·Last reviewed May 3, 2026

Twilio error 21610 fires when the recipient texted STOP, has DND enabled, or never opted in. Resolution: remove the number from your list, or ask them to reply START to opt back in.

Error 21610 from Twilio means your message was blocked because the recipient opted out, enabled Do Not Disturb, or never opted in. The platform enforces this to protect users and keep you compliant with carrier and TCPA rules. The error is not a bug — it is the system doing its job.

What 21610 actually means

The recipient has opted out of messages from one of the following:

  • Your phone number.
  • Your channels sender.
  • Your messaging service.

Or, the contact has DND (Do Not Disturb) enabled in their contact profile in Sparkforge.

If a recipient previously replied STOP, you cannot message them again until they reply START.

Why this happens

  • The recipient texted STOP or another opt-out keyword.
  • The number was reassigned to a new owner who never opted in.
  • The contact has DND enabled for SMS, calls, or all channels.
  • Carrier or compliance rules block messages without verifiable consent.

Fix it

  1. Remove unsubscribed numbers from your list. Filter contacts where DND is on or where opt-out is logged, and exclude them from campaigns.
  2. Ask the recipient to opt back in. They must reply START (or YES) from their handset. You cannot opt them in on their behalf.
  3. Verify consent before sending. Make sure every contact has a clear opt-in event on file — a form submission with an SMS consent checkbox, a verbal confirmation logged in notes, or an opted-in import source.

Opt-out keywords

Twilio blocks messages if the recipient ever replied with any of:

  • STOP
  • STOPALL
  • UNSUBSCRIBE
  • CANCEL
  • END
  • QUIT

Opt-in keywords

To resume messaging, the recipient must reply with one of:

  • START
  • YES

Important

If a contact opts out of a messaging service, the opt-out applies to every number in that service, not just the one they replied to. This protects users from re-messaging through alternate sender numbers.

You are not charged for blocked attempts — Twilio returns the 21610 error and drops the message.

Avoiding 21610 in the first place

  • Capture explicit SMS consent on every form (a checkbox tied to a custom field).
  • Suppress contacts with DND or opt-out tags from any SMS automation.
  • Cleanse imported lists — old numbers may have been reassigned and the new owner never opted in.

Frequently Asked Questions

Will I get charged for messages blocked with error 21610?
No. Twilio returns the 21610 error and the message is not sent, so you are not billed for the attempt.
Can I opt a contact back in on their behalf?
No. Once a recipient texts STOP, only they can opt back in by texting START or YES from the same handset. Manual database changes do not lift the carrier-level block.
Does opting out of one messaging service number affect other numbers?
If the numbers belong to the same messaging service, yes — the opt-out applies to the whole service. Opt-outs from a single non-service number affect only that number.
What are the recognized opt-out keywords?
STOP, STOPALL, UNSUBSCRIBE, CANCEL, END, and QUIT. Replying any of these blocks future messages until the recipient texts START or YES.

Was this article helpful?

Related Articles