Sortof related: I just wanted to add that many MTA's email servers also support this concept. Using milters or scripts, you can default emails to be stored in the hold queue, then release them based on ingestion time.
Cool, I did not know that. Are there any other thinkable use cases of keeping messages in a hold queue for a longer period of time, for this option to exist?
Some sysadmins/postmasters do this to protect against spam floods. They use cron jobs to hold/release emails after validating the envelope contents using tools like MailScanner. Some use that for intercepting test messages [1] I don't have the links handy, but I have also seen some projects in github that utilize hold queues for managing more granular rate limits based on content rather than destination. Some anti-spam tools will also quarantine messages into the hold queue for review.