Some remote SMTP server I am trying to deliver mail to refuses to accept the HELO from my server:

504 5.5.2 <localhost>: Helo command rejected: need fully-qualified hostname

Apparently, my Exim4 server sends localhost as its FQDN. Searching the net and a bunch of config files, I have learned that the value sent as FQDN during HELO is drawn from the primary_hostname configuration variable.

My question is: what is the correct way to change this variable in a Debian system? I guess I can simply hardcode a value in on of the Exim4 config files, but IMHO it would seem to make more sense if the value automagically corresponded to /etc/mailname or some other centralized name config.

I have a feeling that the answer to my question can be found in this text from the Debian wiki:

The name used by Exim in EHLO/HELO is pulled from configuration option primary_hostname. Debian's exim4 default configuration does not set primary_hostname. Exim then defaults to uname() to find the host name. If that call only returns one component, gethostbyname() or getipnodebyname() is used to obtain the fully qualified host name.

If your Exim HELOs as localhost.localdomain, then you have most probably a misconfigured /etc/hosts created by some versions of the Debian installer. In this case, please fix your /etc/hosts.

Unfortunately, I am not familiar enough with Linux server administration to know exacly what all this means :(


Your /etc/hosts file should have at least two records in it. The first record should be of the form:


the second one should be of the form: localhost

You also need to make sure that your /etc/hostname file contains the server's FQDN, and that running hostname -f returns your servers FQDN. If you make sure all of this is correct, and restart Exim, you should start seeing it HELO properly.

  • I have changed /etc/hostname from a local machine name to the FQDN. hostname -f still reports "localhost". The part about /etc/hosts in your answer seems buggy - what is the first record supposed to look like? – Jørn Schou-Rode Jul 27 '09 at 16:08
  • 1
    While hostname -f still reports "localhost", it seems that the change to /etc/hostname did the trick - I am able to deliver my mail now. I still would like to see the missing part of your post though :) – Jørn Schou-Rode Jul 27 '09 at 16:15
  • Oops! Formatting flub. Fixed :-) – Paul Lathrop Jul 27 '09 at 16:37
  • /etc/hostname doesn't typically contain the FQDN on Debian systems, only the short hostname. – Josip Rodin Dec 18 '17 at 20:08

If you use a single configuration file, set the PRIMARY_HOST_NAME variable to the desired name:

For example in /etc/exim4/exim4.conf:

PRIMARY_HOST_NAME = mybox.mydomain.com

or set MAIN_HARDCODE_PRIMARY_HOSTNAME in update-exim4.conf.conf if you are using split file configuration.

To be safe, restart exim.

  • Just a note that the host listed can't be in quotes, otherwise exim sends HELO 'mybox.mydomain.com' which breaks sending mail to Google, perhaps others. – mkomarinski Jul 27 '16 at 15:32
  • I can't find any mention of this "PRIMARY_HOST_NAME" variable name in the documentation. Where did you find this? – Josip Rodin Dec 18 '17 at 20:23

The primary hostname is prompted for during the installation of exim4. You can run the command

sudo dpkg-reconfigure exim4-config

to go through the prompts again.

As far as I know, if you're already using the 'split configuration' (lots of little files under /etc/exim4/conf.d) and opt to stick with that when prompted, dpkg-reconfigure shouldn't overwrite any of your existing config alterations other than to the settings it prompts you for, but, as ever, you should take a backup first to be safe.

  • 1
    It only seems to ask for a "mail name [which] should be the single, fully qualified domain name (FQDN)". However, this setting is correct already, and the HELO still seems to say "localhost". – Jørn Schou-Rode Jul 27 '09 at 16:06

after adding the the same string in /etc/hosts, hostname -f will return your FQDN


The primary_hostname variable is filled according to the fully qualified domain name (FQDN) of the system. On Debian and related systems this is typically formed using the contents of the /etc/hostname and /etc/hosts files. The hostname file should contain the short hostname (e.g. foo), and the hosts file should contain an entry resolving foo into foo.bar.baz, i.e. hostname with its domain suffix (normally matching a DNS entry).

The output from the command hostname (without any parameters) will return the former, and the hostname -f output will return the latter.

The hostname file is typically filled upon installation, and the hosts file can be edited to have one other record in it beside the default localhost records - it should be of the form:

<IP address> <hostname FQDN> <hostname>

The three columns should be separated by spaces or tabs, it doesn't matter. The IP address can be just like localhost, but the hostname part has match the system hostname, and the middle part should be the FQDN, i.e. hostname with a domain suffix.

To verify the line is working, simply run hostname -f and check that it returns your server's FQDN.

Finally, restart Exim, e.g. with:

sudo service exim4 restart

Afterwards you should start seeing it HELO properly. Its variable can be verified by running:

/usr/sbin/exim4 -bP primary_hostname

(NB: I originally did this as an edit to the answer from 2009, but it was rejected. The things I was originally fixing are: there was no need to repeat the default localhost record, just explain what the addition should be; explain which IP address can be used; explain what is FQDN; explain how to verify exim itself.)


I have the same problem. I need to change the mail hostname, because some emails fails, when them was sended. Whith this error:

host receiver.server.com []: 550 <info@sender.server.com>: Sender address rejected: Domain not found

So I change it into the /etc/exim.config the param primary_hostname

Some code of this file:

smtp_connect_backlog = 50
smtp_accept_max = 100

#primary_hostname = myhostname.com 
deliver_queue_load_max = 3
  • Hah, this post helps me with "primary_hostname" and now my exim works better than ever :) – user184004 Nov 6 '14 at 9:35

in WHM goto Exim Configuration Manager -> Advanced Editor

scroll down until you find "Add additional configuration settings" (a big blue button) and click it.

in the new line, add [ primary_hostname ] = [ mydomain.com ]

scroll to the bottom and click Save.

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