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.

[1] - https://serverfault.com/questions/219173/configure-postfix-t...