Let us learn more about the error sendmail host map lookup deferred and how to remove it with the support of our Server management support services at Bobcares.

Error: sendmail host map lookup deferred

The error message might appear while using the sendmail email server.

Sendmail was unable to resolve a hostname or domain name supplied in the email message, as indicated by this error message.

A DNS (Domain Name System) problem is the most prevalent reason of this error. DNS is utilized by Sendmail to convert domain names into IP addresses, which are then used to route email messages to the right destination.

Resolve: sendmail host map lookup deferred

We may attempt the following procedures to resolve the error:

1. Check that the DNS server is operational: To see if the DNS server can resolve the domain name in question, use the nslookup command.

   For example, to verify a domain’s MX record, execute the following command:

   nslookup -type=mx example.com

   This command should produce a list of the domain’s mail servers. If it does not, there might be a problem with the DNS server.
2. Examine the Sendmail setup:

   Check to see sure the Sendmail settings is proper and up to date. To utilize a new DNS server or to provide a different hostname or domain name, we may need to edit the Sendmail configuration file.
3. Examine the firewall settings:

   Check to see whether the firewall is preventing outgoing DNS requests. If the firewall is preventing outgoing DNS requests, we may need to adjust it to enable them.
4. Examine the network setup as this may cause sendmail host map lookup deferred:

   Check the network settings to ensure that your server has a valid IP address and gateway. If the server cannot connect to the network, it will be unable to resolve domain names using DNS.
5. Disabled DNS lookup for local sendmail, and received the following sendmail parameter:

   define(`confSERVICE_SWITCH_FILE',`/etc/mail/service.switch')dnl

   And only one line in this file:

   cat service.switch
hosts files

   As a result, the local sendmail would only query hostanme in the /etc/hosts file.

   We can rebuild the sendmail cf files and restart it after such a change, and it works well.
6. Replace the line given below:

   define(`confFALLBACK_SMARTHOST', `mailhost$?m.$m$.')dnl

   To the line given below:

   define(`confBIND_OPTS', `WorkAroundBrokenAAAA')dnl

These steps will clear out the host map lookup deferred error.

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Conclusion

To sum up we have now seen how to remove the sendmail host map lookup deferred with the support of our tech team.