Exim

Posted: 2010-03-07 23:40:34 by Alasdair Keyes

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For the past 4 years or so, I've used Sendmail as my MTA of choice, no real reason for it except that it was the default on most Linux distros. However, over the same time, all the mail systems I've had to maintain professionally have been Exim. I'm in the process of migrating my dedicated server over to a Daily VPS solution... mainly because I created the whole system so I trust it :)

Anyway, I've decided that instead of using Sendmail, which is pretty horrid, clunky and not easily (in my opinion) extendible, I'd change my MTA to Exim and Dovecot.

Although MTAs are a necessary evil as email's not going anywhere... I hate them, I hate MTAs, I hate configuring them, I hate tweaking them, and although I think Exim's configurability is outstanding, it can be a real pain setting up a mail server so I thought I'd outline how to setup a basic Exim mail server (Under CentOS 5). As I only have ~20 domains which don't change very often, I've decided to stick with file-based mail configuration, you can use a SQL backend, but there's no need for it on my system. This setup will hold mailboxes/forward and allow users to send mail. So...

Install exim and if necessary remove any other MTA on the machine (Sendmail,Postfix,etc). Also install saslauthd for authentication and dovecot for mail collection.

yum remove sendmail -y;
yum install exim dovecot saslauthd -y

Create a folder to hold the mail routing information on a per-domain basis. In this folder we will create files with the same name as the domains we wish to handle mail for and in each file we will place the mail routing information

mkdir /etc/exim/mail_configs
chown root:mail /etc/exim/mail_configs

Create the file /etc/exim/mail_configs/example.com to hold information for a domain example.com

al : al@localhost
* : :fail: Unknown User
group : al[at]gmail[dot]com,al[at]hotmail[dot]com

The above tells it to deliver al@example.com to the localuser al. To forward group@example.com to a Hotmail and Gmail address and reject all other addresses. That's all that's required for the per-domain setup, now we just have to configure Exim.

In /etc/exim/exim.conf change the following...

Tell exim that all the names of all the files in /etc/exim/mail_configs should be considered the domains we handle locally

domainlist local_domains = @ : localhost : localhost.localdomain

to

domainlist local_domains = @ : localhost : localhost.localdomain : dsearch;/etc/exim/mail_configs

Tell Exim how to find local users, place this block as the first entry in Exim's router configuration

my_aliases:
      driver = redirect
      allow_defer
      allow_fail
      domains = dsearch;/etc/exim/mail_configs
      data = ${expand:${lookup{$local_part}lsearch*@{/etc/exim/mail_configs/$domain}}}
      retry_use_local_part
      pipe_transport   = address_pipe
      file_transport   = address_file
      no_more

Tell Exim to allow plaintext authentication when users send emails through the server. Enter this under the begin authenticators section of exim.conf

begin authenticators

PLAIN:
  driver                     = plaintext
  server_set_id              = $auth2
  server_prompts             = :
  server_condition           = ${if saslauthd{{$2}{$3}{smtp}} {1}}
  server_advertise_condition = ${if def:tls_cipher }

LOGIN:
  driver                     = plaintext
  server_set_id              = $auth1
  server_prompts             = <| Username: | Password:
  server_condition           = ${if saslauthd{{$1}{$2}{smtp}} {1}}
  server_advertise_condition = ${if def:tls_cipher }

Because we're using plaintext, force users who want to send mail to use TLS otherwise they'll just get a relay denied error. Enter this under the main exim config section

auth_advertise_hosts = ${if eq {$tls_cipher}{}{}{*}}

By default Exim will store messages in /var/mail/$user as a regular spool. I want to use Maildir storage so Change the local_user transport section to use maildir

local_delivery:
  driver = appendfile
  file = /var/mail/$local_part
  delivery_date_add
  envelope_to_add
  return_path_add
  group = mail
  mode = 0660

to

local_delivery:
  driver = appendfile
  directory = $home/Maildir
  maildir_format
  maildir_use_size_file
  delivery_date_add
  envelope_to_add
  return_path_add

That's Exim sorted. Now we tell saslauthd to look at the /etc/shadow file for authentication and not PAM. Edit /etc/sysconfig/saslauthd Change MECH=pam to MECH=shadow

Finally tell Dovecot that we're using Maildir and not mbox. Of course this step isn't necessary if you want to use mbox. Edit /etc/dovecot.conf and set

mail_location = maildir:~/Maildir

Restart the lot

service exim restart;
service saslauthd restart;
service dovecot restart;

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