Unless both you and the reicpient are using Microsoft Exchange Server
accounts and Outlook, you can't. It has passed out of your control.
Now might be the time to call and apologise.
Since you ask, the only other way would be to gain access to their mail, and
delete the message, however that may have other ramifications including
legal ones.
Outlook with Exchange Server, and possibly some other corporate-type mail
servers such as Notes, allow a "recall" feature that works until the message
has been read. But, it only works for those who are also on the Exchange
server; it does not work for Internet Mail servers.
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