Can sendmail be used without DNS?

Solution Verified - Updated

Environment

  • Red Hat Enterprise Linux
  • sendmail

Issue

  • How to use sendmail without DNS?
  • Can I setup sendmail to forward outgoing mails to a different system (smarthost), without having DNS available?

Resolution

There are a number of steps required to successfully use sendmail when there is limited or no DNS.

  1. 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

    • For more information, please see: How to configure a Sendmail Smart Host

  2. 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

  3. Ensure that the ServiceSwitchFile (by default at /etc/mail/service.switch) has content similar to:

    aliases files
hosts files

  4. 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

Diagnostic Steps

  • Various logfiles in directory /var/log can help debugging mail issues.
  • To send testmails commands like the following can be used:
# echo "content" | mailx -s "subject" recipient@receiverdomain.example.org
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