21611: This 'From' number has exceeded the maximum number of queued messages

Twilio returns 21611 when you try to create another outbound message for a From sender whose queue is already full. Twilio queues outbound messages according to the throughput available to a sender or account, and a queue can hold up to 10 hours of message segments. If you send with a Messaging Service, sender pool overload can instead appear after message creation as error 30001.

• Your application is submitting messages for one From sender faster than Twilio can dequeue them at that sender's available throughput. (help.twilio.com)
• The queued volume for that sender has grown beyond the maximum queue window, which is up to 10 hours of message segments. (help.twilio.com)
• You set a lower Validity Period on the message request or Messaging Service, which reduces how long messages can remain queued before they fail.

• Reduce the rate at which your application submits messages for that sender so the queue can drain.
• Use a Messaging Service when it fits your use case so Twilio can distribute traffic across an appropriate sender pool.
• Review the Validity Period on the message request or Messaging Service. If you set it below 36,000 seconds, increase it as needed up to the 10 hour maximum.
• Monitor queue time, delivery failures, and backlog trends in Messaging Insights or Messaging Logs so you can detect congestion before new requests start failing.

• Understanding Twilio Rate Limits and Message Queues
• Messaging Services
• Messaging Insights Dashboards