I'm half a moron.
I need to be able to edit sent Moderator Private Messages because I cannot stop pressing the send button before proofreading my messages.
Please, think of the morons.
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I want to be able to edit emails that I've sent to people, after those people have already read them.
As Grace Notes, mod messages send emails by default, although you can disable some of them. While that doesn't prevent us from implementing some sort of editing feature for the on-site messages, doing so reinforces the wrong idea: mod-messages are intended to be important and rare, worth getting right the first time. You should be pausing before sending, dread building up in the pit of your stomach as you think of all the things that could go wrong. A typo can be forgiven, whether corrected or not; a message that pours gas on a fire will sit in a user's inbox regardless of how much you regret it later.
A compromise solution would be to allow editing if an email hasn't been sent, and if the recipient hasn't read the message on-site yet. Donno if this is worth the complexity though.
Why not add a popup to confirm you have proof read it before submitting the form. You can implement it as a script and post it on stack apps.