SMTP was first implemented when Internet was a network much different than the one we use today. Relaying back then was not frowned upon, as it was a legitimate and sometimes necessary way of sending messages to or from hosts that were not constantly connected. RFC 821 provides even specific commands for relaying, in the form
RCPT TO:<@HOSTX.ARPA:JOE@HOSTW.ARPA>This is the first point we should note -- This syntax should not be allowed by the wrapper. Current Sendmail versions usually rejects this syntax, but it should not be overlooked.