Reader Prerequisites: To get the most from this article, you should understand the following concepts before reading: basic Core Utilities, configuration files, text editors, DNS, hostname, email
If your system pauses for a long time during bootup at the message,
then this will fix the issue.
The problem occurs because the email program Exim4 is doing a DNS query on the localhost name. It will do this even if you have an entry in the /etc/hosts file for your local host. Why? Because Exim4 is doing an IPv6 lookup and the entry in your /etc/hosts file is most likely an IPv4 entry. Hopefully, this bug will be eliminated in the next release of Debian. Until then, there are many ways to correct the problem:
#1 - Quickie, HOW-TO, Step-by-Step Method
If you want to change your hostname, you can just change it to a FQDN (Fully Qualified Domain Name). Here's how:
1. Edit the file /etc/hostname and change the hostname from the localhost to the localhost.domainname.tld
If you don't have a domainname of your own, you can make one up. For example, if your hostname is mycomputer, you can change it to mycomputer.dummy.com. After you save the /etc/hostname file,
2. Type hostname -F /etc/hostname to register the changes.
#2 - Everybody who likes to learn something method
To prevent the DNS lookups at the source, you need to change one of the configuration files for Exim4. Unfortunately, Exim4 has one of the most convoluted configuation schemes in existence. Before you start, read the text document at /usr/share/doc/exim4/README.Debian. It's a long document, but there is no need to understand it all. Concentrate on the part about how the configuration files work.
Now you need to change the Exim4 configuation file at /etc/exim4/update-exim4.conf.conf (yes, that's the name with two ".conf" on the end of it!) In the file, find the line that says
Change the line to read:
Then save the file and restart Exim4 (or reboot your computer). From now on, it won't do those DNS lookups during boot anymore.
If one of the above methods causes problems somewhere else in your system, there are several other methods to correct the problem. These methods are briefly described in /usr/share/doc/exim4/README.Debian in the FAQ section - Runtime Issues.