In the modern Internet, the first step in mail delivery is to parse each recipient address, and perform a DNS lookup on the domain portion. Thus baccala@FreeSoft.org will trigger a DNS lookup on FreeSoft.org. The lookup will be first for an MX (mail exchanger) record, then a normal A (address) lookup if this fails. MX records contain a preference value (to chose among multiple MX records for a particular domain), and the DNS name of the host receiving mail for this domain. This entire procedure is documented in RFC 974.