Mail server systems use the DNS -- Domain Name System. When one is about to deliver a piece of e-mail, it looks at the address and queries the DNS (there are system calls to do this) to determine to what system(s) to send the e-mail. If the e-mail should go to a mail exchanger (MX), as most Internet e-mail does today, the MX record for the address will point to the systems that claim to accept mail for the particular domain.
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