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

You have attempted to enqueue too many messages for a given 'From' number. Twilio SMS sends messages at 1 message per second from a phone number and 30 messages per second from a shortcode. Messages that exceed these rates will be queued and delivered at the defined rates. When a queue for a particular 'From' number exceeds 4 hours, this error will be thrown.