Hi there and welcome
You cannot sorry, you have about 6 seconds when you send, to click undo else it has left the email server and entered the recipients inbox. Once it is in the inbox, the only way would be to hack into the account and delete it.
Its different to a companies mail server where recalling is possible. But when you do it via Gmail, you have 6 seconds as it has a delay for this, but once this is over, it is too late sorry.
If you would like any additional information or assistance, please do not hesitate to let me know.
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