To enable a specific host to relay through your sendmail configuration, add its hostname to the /etc/mail/relay-domains file. Normally, entries in this file represent entire domains which are allowed to relay. However, if you want to list specific hosts in this file and not subdomains, you can use FEATURE(relay_hosts_only).
If for some reason you need to configure sendmail to relay all traffic it encounters, the FEATURE(promiscuous_relay) macro causes the old behavior to occur, where sendmail will relay anything. However, using this macro is strongly discouraged, since it will allow spam to be relayed through your mail server.
