SQS (Simple Queue Service)
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What is it?
A service for queuing messages of any kind to be processed later.
Why did we choose it?
It's a necessary part of good practice when using SES (the email service).
What do we use it for?
Receiving notifications from SNS when an email fails to deliver. SNS sends the message to SQS, and then we periodically check the SQS queue for any messages indicating undelivered emails.