If you received Error 21611, it means that you have attempted to enqueue too many messages for a given "From" number. This error will be thrown when attempting to create additional messages on the same number.
Messages can only be queued for 4 hours - so they automatically fail. If your outbound messages are queueing at a higher rate, your messages will accumulate. After sitting in the queue for 4 hours - those messages will fail and won't be sent.
You have attempted to enqueue more than 4 hours' worth of messages from a single number
Slow down your sending rate to avoid queuing on your Aloware number
If appropriate for your use case, consider using a Messaging Service to better distribute message throughput.