This sendmail(1M) message indicates that the destination host machine, specified by the portion of the address after the at-sign (@), was not found during domain naming system (DNS) lookup.
Use the nslookup(1M) command to verify that the destination host exists in that or other domains, perhaps with a slightly different spelling. Failing that, contact the intended recipient and ask for a proper address.
Sometimes this return message indicates that the intended host is inoperable, rather than unknown. If a DNS record contains an unknown alternate host, and the primary host is inoperable, sendmail(1M) returns a "Host unknown" message from the alternate host. [This is a known sendmail(1M) version 8.6.7 bug.]
For uucp(1C) mail addresses, the "Host unknown" message probably means that the destination host name is not listed in the /etc/uucp/Systems file.