Sendmail: “unqualified hostname unknown; sleeping for retry unqualified hostname

If you have configured Sendmail on your Linux system and are still unable to send emails because of a unqualified hostname error, we have a few suggestions that you can use to try and remedy the issue.

In this tutorial you will learn:

  • How to solve unqualified hostname error in Sendmail
Sendmail unqualified hostname error
Sendmail unqualified hostname error
Software Requirements and Linux Command Line Conventions
Category Requirements, Conventions or Software Version Used
System Any Linux system
Software Sendmail
Other Privileged access to your Linux system as root or via the sudo command.
Conventions # – requires given linux commands to be executed with root privileges either directly as a root user or by use of sudo command
$ – requires given linux commands to be executed as a regular non-privileged user

Sendmail unqualified hostname error

The Sendmail error will look something like this:

debian sm-mta[8129]: My unqualified host name (debian) unknown; sleeping for retry

Other symptoms incude Sendmail being very slow or hanging completely.

This error output actually makes the problem quite clear. The unqualified host name text means exactly what it says. It means that Sendmail is not able to resolve your fully qualified domain name.

In our example, our system’s hostname is debian. You can check yours with the hostname command.

$ hostname

To begin troubleshooting, check the contents of your /etc/hosts file. In our case the host name is “debian” is not a FQDN. To resolve this problem change /etc/hosts:

From:       localhost       debian

To:       localhost.localdomain localhost debian       debian

Of course, substitute debian with the actual name of your FQDN.

After you have made these changes, you can restart the sendmail service and try sending your email again.

$ sudo systemctl restart sendmail

Even with this fix, your email is unlikely to be delivered if you do not have a properly configured fully qualified domain name. This is a presrequisite that must be setup before you can expect sendmail to work. Such a setup would be outside the scope of this tutorial. However, you can always follow our guide on Configuring Gmail as a Sendmail email relay in order to use gmail or your own Google Apps account to send email from the Linux command line.

Closing Thoughts

In this tutorial, we learned about some troubleshooting tips for the sendmail unqualified host name error on Linux. It basically boils down to a simple change inside the /etc/hosts file, but this assumes that you have already purchased and configured your fully qualified domain name correctly. If this is out of your depth, it is also possible to configure your gmail account as a sendmail relay, as linked above.

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