Or you can block it using SpamBlocker, built into DirectAdmin.
Add the hostname to /etc/virtual/bad_sender_hosts or the domain (right side of the @ sign) to /etc/virtual/blacklist_domains. Then make sure that the recipient address domain is in /etc/virtual/use_rbl_domains, or that /etc/virtual/use_rbl_domains is a link to /etc/virtual/domains.
Lots of information on SpamBlocker in these forums.
Jeff