how do I unsend an email?

As it is already sent, there is no way to recall it. With the current speed of the Internet, the message was likely delivered prior to the thought that you wanted to recall it had formed in your brain.

If your email is hosted by an internal network such as an Exchange server, the function can be implemented, but as I mentioned before, the network speeds make such a thing impractical. The only way it could possibly be recalled is if nobody had read the message, and the ability to know this would only be possible on some closed network.

Unless the documentation for your email service says otherwise, you can't.

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