There are a number of steps required to successfully use sendmail when there is limited or no DNS.
•Setting a SMARTHOST
◦Ensure the required host is resolvable, either by /etc/hosts or DNS, or alternatively specify an IP address.
◦Use line in /etc/mail/sendmail.mc that resembles define(`SMART_HOST',`name.of.smart.host')dnl
•Since the system implicitly have limited resolving capabilities, accept email for unknown domains
◦Use line in /etc/mail/sendmail.mc of the form FEATURE(accept_unresolvable_domains)dnl
•Ensure that the ServiceSwitchFile (by default at /etc/mail/service.switch) has content similar to:
•Setting the submission agent to ignore DNS
◦Use line in /etc/mail/submit.mc of the form define(`confDIRECT_SUBMISSION_MODIFIERS',`C')
◦Use line in /etc/mail/submit.mc of the form FEATURE(accept_unresolvable_domains)dnl
Having completed the above changes remake the submit.cf & sendmail.cf by typing make in /etc/mail
The daemon may also need to be restarted with the command:
# service sendmail restart