Diagnostic-Code: SMTP; 550 host unknown
When we’re trying to send mail to localdomain through mailx from linux box. It says “Diagnostic-Code: SMTP; 550 host unknown” and mail not sent.
Solution :
Edit sendmail.cf
# “Smart” relay host (may be null)
DS[10.x.x.xx]
restart the sendmail service and send the mail is successful
10.x.x.xx — mailbox server
DBA 