This is a nice way to show what you can do with Postfix. But if you actually wanted this functionality, it would be much less error prone to implement this in your delivery agent rather than your transfer agent. For example Procmail or Sieve or whatever you use to filter incoming mail.

(An email system is normally in three parts, a transfer agent that talks to remote systems such as Postfix, a delivery agent that delivers to a local mailbox such as Sieve or Procmail, and a user agent which is what you use to read your mail such as Thunderbird or mutt.)

Just change your filter to change the delivery mailbox to a separate one for weekends. That way you won't risk causing any bounces if you do something wrong.

This would be really popular for sms, especially for mac interface or iphone.

E-mail is, at its core, best-effort, respond-maybe protocol. Implementing "quiet hours as seen in IM", means using the wrong tool.

If you insist on treating e-mail as a medium requiring immediate response, you'll be going into Postfix on weekends to force-flush the queue instead. This is not a technological problem, definitely not one that's solvable through configuration tweaking.

(I mean, sure, it's an impressive exercise in how flexible the sw stack is, but the issue lies elsewhere)