Twilio Error 21610 "The message from/to pair violates a blacklist rule" when sending SMS

Modified on: Tue, 23 Sep, 2025 at 3:59 AM

Overview

When sending SMS/MMS through Twilio (or integrated platforms), you may encounter cases where your message cannot be delivered because the recipient has opted out or enabled Do Not Disturb (DND). This is enforced to honor user preferences and maintain compliance with messaging regulations.

Description

The person you are trying to message has opted out of receiving messages from:

Your phone number,
Your Channels sender, or
Your Messaging Service.

Alternatively, the contact may have DND (Do Not Disturb) status enabled in their contact profile.

If a recipient has previously replied with “STOP” to one of your messages, you will not be able to send to them again until they respond with “START.”

Possible Causes

The recipient’s handset replied with STOP or another opt-out keyword.
The phone number may have been reassigned to a new user who never opted in.
The contact has enabled DND for SMS, calls, or all communication types.
The platform blocks outgoing messages to respect regulatory and compliance requirements.

Possible Solutions

Remove unsubscribed numbers from your contact list.
Ask the recipient to re-subscribe by texting “START” (or another opt-in keyword) to your number.
Verify consent before sending — ensure recipients have explicitly opted in to receive your messages.

Twilio-Specific Opt-Out Behavior

If you attempt to send a message to a recipient who has opted out, the Twilio API will return an error, and the message will not be sent. You will not be charged for the attempt.

Opt-Out Keywords (unsubscribe)

Messages will be blocked if the recipient replied with any of these:

STOP
STOPALL
UNSUBSCRIBE
CANCEL
END
QUIT

Opt-In Keywords (resubscribe)

To resume receiving your messages, recipients must reply with:

START
YES

Important Notice

If a user opts out of a Messaging Service number, the opt-out applies to all messages sent from that service.
This ensures subscribers are fully protected from unwanted communication.

Key Takeaway: Always secure proper opt-in consent, and respect opt-out requests. If someone has opted out, they must manually opt back in before you can send them messages again.

Reference- https://support.twilio.com/hc/en-us/articles/223133627-Error-21610-The-message-From-To-pair-violates-when-sending-SMS