Error 21610 occurs when you try to send a message to a recipient who has opted out of receiving messages or has Do Not Disturb settings enabled. Messaging remains blocked until the recipient properly opts back in.

TABLE OF CONTENTS


What Is Error 21610?

Error 21610 means the destination number is currently blocked from receiving messages from the sender.

This commonly happens after the recipient replies with STOP or another recognized opt-out keyword. The contact may also have Do Not Disturb enabled for SMS or broader communication channels.

The block is enforced to respect the recipient’s messaging preferences and help maintain compliance with consent requirements.


Key Benefits of Respecting Opt-Out Status

Honoring opt-out requests helps protect recipients and keeps messaging activity compliant.

  • Respect Recipient Preferences: Stops messages after a contact withdraws consent.

  • Reduce Compliance Risk: Prevents continued messaging to contacts who have opted out.

  • Improve Contact Data Quality: Keeps recipient communication preferences current.

  • Prevent Repeated Delivery Failures: Avoids repeatedly attempting messages that are blocked.


Common Causes of Error 21610

Error 21610 may occur when:

  • The recipient replied with STOP.

  • The recipient used another recognized opt-out keyword.

  • SMS Do Not Disturb is enabled for the contact.

  • Do Not Disturb is enabled for all communication channels.

  • The phone number was recently reassigned to another person who previously opted out.

  • A workflow or campaign is attempting to send to a contact whose messaging permission has been withdrawn.


How to Fix Error 21610

Start by reviewing the contact’s messaging status and recent conversation history.

  1. Open the affected contact.

  2. Review recent inbound messages for STOP or another opt-out keyword.

  3. Check whether Do Not Disturb is enabled for SMS or all communication types.

  4. Do not continue sending messages while the contact remains opted out.

  5. Remove the contact from active messaging campaigns or workflows if needed.

  6. If the recipient wants to receive messages again, ask them to send START or another supported opt-in keyword from their own device.

  7. Confirm that the opt-out or DND status has been removed before sending again.


Check the Contact’s Opt-Out History

A recipient may have previously withdrawn consent even if the opt-out happened some time ago.

Review:

  • Conversation history

  • SMS status

  • DND settings

  • Workflow enrollment

  • Campaign participation

If the contact opted out, do not manually work around the block.

How a Recipient Can Opt Back In

The recipient must initiate the opt-in when required.

A common method is for the recipient to reply with:

START

After the supported opt-in keyword is received, messaging may become available again.

Do not send messages asking the recipient to bypass an active opt-out through another messaging route unless you have an appropriate, consent-based reason to contact them.


Check Do Not Disturb Settings

Error 21610 can also occur when Do Not Disturb is enabled.

Review whether DND applies to:

  • SMS only

  • Calls

  • All communication types

If DND was enabled because of a recipient opt-out, do not manually disable it unless the recipient has properly opted back in.


Check Workflows and Campaigns

Automated messaging can continue attempting delivery even after a contact opts out unless recipient eligibility is handled correctly.

Review:

  • Active workflows

  • Bulk campaigns

  • Follow-up sequences

  • Scheduled SMS

  • Contact filters

Remove or suppress opted-out contacts from messaging activity.


Frequently Asked Questions

Q: What does Error 21610 mean?
It means the recipient has opted out of messages or has Do Not Disturb enabled.

Q: Can I manually remove the opt-out and send again?
Do not override a legitimate recipient opt-out. The recipient should properly opt back in before messaging resumes.

Q: How can the recipient opt back in?
The recipient can typically send START or another supported opt-in keyword from their phone.

Q: Can DND cause Error 21610?
Yes. SMS or account-wide Do Not Disturb settings can prevent delivery.


Q: What if the phone number was reassigned to someone else?
A newly reassigned number may inherit an existing opt-out condition. Confirm current consent before messaging the new owner.

Q: Should I remove opted-out contacts from campaigns?
Yes. Opted-out contacts should not continue receiving SMS through campaigns or automated workflows.


Need Help?

If Error 21610 continues, first review the contact’s opt-out and Do Not Disturb status.

Verify that:

  • The recipient has not replied with STOP or another opt-out keyword.

  • The contact has valid consent to receive messages.

  • SMS Do Not Disturb is not enabled.

  • Do Not Disturb is not enabled for all communication channels.

  • The contact is not still enrolled in campaigns or workflows that should exclude opted-out recipients.

  • The recipient has sent START or another supported opt-in keyword if they want to resubscribe.

  • You are not manually overriding a legitimate opt-out.

  • The phone number has not recently been reassigned to a different person.

  • The contact’s messaging status reflects the recipient’s current preference.

If the recipient has opted out, do not continue sending messages until they properly opt back in.

If the issue continues after the recipient has opted back in, collect the destination number, opt-out status, DND status, recent inbound keyword, workflow or campaign name if applicable, message timestamp, and a screenshot of Error 21610 before contacting support.