DSN 5.0.0 – Service Unavaliable

Ran into this error today on a server. Some mails got through but some was not getting through.

Check that /etc/hosts has a FQDN (fully qualified domain name).

Also check the size of the mails being sent, in my case the issue was that mail was to large for the receiving server thus this error message was produced at the sender.

unqualified host name ([something]) unknow; sleeping for retry

sendmail expects to the machine to have a FQDN (fully qualified domain name).

If it does not have that (type “hostname” to check what name the machine has) sendmail will complain about this and take a loot of extra time trying to find it.

To use a FQDN edit “/etc/hosts” and change [something] to [something.yourdomain.com] and this will go away. Updating “/etc/hostname” at the same time might be a good idea; don’t forget to run “/bin/hostname -F /etc/hostname” after updating /etc/hostname.