Many developers encounter this common Sendmail error when their mail server can't properly resolve the local hostname. The error message typically appears in /var/log/maillog:

"My unqualified host name (foo.bar) unknown; sleeping for retry"

Sendmail requires a fully qualified domain name (FQDN) for proper operation. When it only finds an unqualified hostname like "foo.bar" (without a domain suffix), it throws this warning and may delay mail sending.

While adding the hostname to /etc/hosts is often suggested, the solution requires more comprehensive configuration:

127.0.0.1     localhost localhost.localdomain
127.0.0.1     foo.bar foo.example.com

First, verify your current hostname:

hostname
hostname -f

Then update these critical files:

# /etc/hosts
127.0.0.1   localhost localhost.localdomain
192.168.1.10 foo.example.com foo

# /etc/hostname
foo.example.com

# /etc/mail/local-host-names
foo.example.com
localhost.localdomain
localhost

Edit your Sendmail configuration file (/etc/mail/sendmail.mc) and add:

define(confDOMAIN_NAME', foo.example.com')dnl
FEATURE(use_cw_file')dnl

Then rebuild the configuration:

make -C /etc/mail
service sendmail restart

Check if Sendmail now recognizes your hostname:

sendmail -d0.1 -bv root | grep Canonical

For WordPress specifically, you might need to adjust the PHP mail configuration:

# /etc/php.ini (or relevant PHP config file)
[mail function]
sendmail_path = /usr/sbin/sendmail -t -i -f webmaster@example.com

If problems continue, check DNS resolution:

dig foo.example.com
nslookup foo.example.com

And verify Sendmail's mail queue:

mailq

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When working with sendmail or mail-related plugins (like WordPress email plugins), you might encounter this error in your maillog:

"My unqualified host name (foo.bar) unknown; sleeping for retry"

This typically occurs when Sendmail can't properly resolve your server's hostname to a fully qualified domain name (FQDN).

Sendmail requires a properly configured hostname that meets these criteria:

• Must be a FQDN (like server.example.com)
• Must be resolvable both forward and reverse
• Must appear in your /etc/hosts file correctly

First, check your current hostname:

hostname
hostname -f

Then examine your /etc/hosts file:

cat /etc/hosts

Here's how to properly configure your system:

1. Set a FQDN Hostname

sudo hostnamectl set-hostname server.example.com

2. Correct /etc/hosts Configuration

Your /etc/hosts should look like this:

127.0.0.1   localhost localhost.localdomain
127.0.1.1   server.example.com server

3. Update Sendmail Configuration

Edit /etc/mail/sendmail.mc and ensure these lines exist:

define(confDOMAIN_NAME', server.example.com')dnl
define(confCF_VERSION', server.example.com')dnl

Then rebuild the sendmail configuration:

sudo make -C /etc/mail

After making changes, test with:

sendmail -bv root

You should see output confirming proper delivery configuration.

If you need a temporary solution, add this to /etc/mail/local-host-names:

foo.bar
localhost
localhost.localdomain

Then restart sendmail:

sudo systemctl restart sendmail

• Using just 127.0.0.1 without the FQDN
• Not having reverse DNS properly configured
• Forgetting to rebuild sendmail config after changes