Use Exim With Multiple Smarthosts

Abstract

userx@example.com:   user=usera   smart_host=mail.example.com   auth_name=u113   auth_pass=secret

• A user called usera has somewhere the email-address userx@example.com.
• Mails to userx@example.com are no longer delivered to this address but directly to the local user usera.
• Mails from userx@example.com are delivered over the smarthost mail.example.com with the user-name u113 and the password secret (cram_md5 encrypted).

userx@example.com:                smart_host=mail.example.com   auth_name=u113   auth_pass=secret

• Now mails to userx@example.com are really delivered to this address.

userx@example.com:   user=usera   smart_host=mail.example.com   auth_plain=^u113^secret

• Now we don't use cram_md5 to encrypt the password but send the it plain-text.

userx@example.com:   user=usera                                                          

• Here we don't use a smarthost but deliver the mails directly to the destination mailserver.

Define Smarthost

1. The simplest variation is to deliver mails directly. The achieve this write this entry in your "ROUTERS CONFIGURATION" section:
   

   # direct delivery
   smarthost_directly:
     debug_print = "R: dnslookup for $local_part@$domain"
     driver = dnslookup
     domains = ! +local_domains
     transport = remote_smtp
     #same_domain_copy_routing = yes
     #ignore_target_hosts = 0.0.0.0 : 127.0.0.0/8 : 192.168.0.0/16 : 172.16.0.0/12 : 10.0.0.0/8 : 169.254.0.0/16
     no_more

2. Alternative you may define a smarthost that delivers all your mail. If your provider's smarthost is called mail.example.com use this entry:
   FIXME
   

   smarthost:
      driver = domainlist
      transport = remote_smtp
      route_list = "* mail.example.com bydns_a"

3. Some smarthosts only accept their own addresses. This is how you distribute your mails to different smarthosts according to their sender-domain:
   FIXME
   

   smarthost_alpha:
      condition = ${if eq {${lc:$sender_address_domain}} {example.com} {true} fail }
      driver = domainlist
      transport = remote_smtp
      route_list = "* mail.example.com bydns_a"
   
   smarthost_beta:
      condition = ${if eq {${lc:$sender_address_domain}} {example.org} {true} fail }
      driver = domainlist
      transport = remote_smtp
      route_list = "* smtp.example.org bydns_a"
   
   ...

4. You may even have a different smarthosts for each sender-address:
   FIXME
   

   smarthost_alpha:
      condition = ${if eq {${lc:$sender_address}} {usera@example.com} {true} fail }
      driver = domainlist
      transport = remote_smtp
      route_list = "* mail.example.com bydns_a"
   
   smarthost_beta:
      condition = ${if eq {${lc:$sender_address}} {userb@example.com} {true} fail }
      driver = domainlist
      transport = remote_smtp
      route_list = "* mail.example.com bydns_a"
   
   ...

5. If you use the configuration file I presented at the beginning of this page use this to let exim automatically decide if and which smarthost to use:
   

   smarthost_auto:
      condition = ${extract{smart_host}{${lookup{$sender_address}lsearch{/etc/exim/smtp_users}{$value}fail}}}
      driver = manualroute
      domains = ! +local_domains
      route_list = "* ${extract{smart_host}{${lookup{$sender_address}lsearch{/etc/exim/smtp_users}{$value}fail}}} bydns_a"
      transport = remote_smtp

   # direct delivery
   smarthost_directly:
     debug_print = "R: dnslookup for $local_part@$domain"
     driver = dnslookup
     domains = ! +local_domains
     transport = remote_smtp
     #same_domain_copy_routing = yes
     #ignore_target_hosts = 0.0.0.0 : 127.0.0.0/8 : 192.168.0.0/16 : 172.16.0.0/12 : 10.0.0.0/8 : 169.254.0.0/16
     no_more

Address Rewriting

• The following entry delivers all mails to somemachinename@example.com to server@example.com:
  

  *@*.example.com      $1@server.example.com    T

• Users of my configuration file may use this entry to deliver mails to users instead of the users public email-address if you have a matching line in the configuration file:
  

  *@* "${extract{user} {${lookup{$0}lsearch{/etc/exim/smtp_users}{$value}fail}} {$value} fail}@localhost" T

Authentication

• Enable authentication for hosts host1.example.com, host2.example.net and host3.example.org (you have to list all smarthosts where Exim may authenticate users):
  

  remote_smtp:
    debug_print     = "T: remote_smtp for $local_part@$domain"
    driver = smtp
    hosts_try_auth  = host1.example.com:host2.example.net:host3.example.org

• SMTP-Auth with the name "usera" and the password "secrect" (cram_md5 encrypted):
  

  cram_md5:
       driver = cram_md5
       public_name = CRAM-MD5
       client_name = usera
       client_secret = secrect

• SMTP-Auth with the name "usera" and the password "secrect" (plaintext):
  

  plain:
       driver = plaintext
       public_name = PLAIN
       client_send = ^usera^secrect

• Or get name, password and send method automatically from my config-file:
  

  cram_md5:
       driver = cram_md5
       public_name = CRAM-MD5
       client_name = "${extract{auth_name}{${lookup{$sender_address}lsearch{/etc/exim/smtp_users}{$value}fail}}}"
       client_secret = "${extract{auth_pass}{${lookup{$sender_address}lsearch{/etc/exim/smtp_users}{$value}fail}}}"
  plain:
       driver = plaintext
       public_name = PLAIN
       client_send = "${extract{auth_plain}{${lookup{$sender_address}lsearch{/etc/exim/smtp_users}{$value}fail}}}"

Convenient Options

• ###
  # MAIN CONFIGURATION SETTINGS
  ###

• Only accept mails where all headers's syntax is error-free. (beware: OE and Bcc recipients):
  FIXME (gibts nicht mehr?)
  

  headers_check_syntax

• Only accept SMTP connections from localhost. This might suffice if you receive your mails with POP3:
  

  hostlist relay_from_hosts = 127.0.0.1 : ::::1
  ...
  begin acl
  ...
  accept hosts = +relay_from_hosts

• Additionally accept mails from 10.0.* addresses:
  

  hostlist relay_from_hosts = 127.0.0.1 : ::::1 : 10.0.0.0/16
  ...
  begin acl
  ...
  accept hosts = +relay_from_hosts

• Allow some trusted users to change their sender-address:
  

  trusted_users = mail:user1:user2

• Always send a copy of error-message-mails to the postmaster:
  

  freeze_tell_mailmaster
  errors_copy = "*@*  postmaster"

• If you don't use this option some mailing-list software might reveal all Bcc recipients:
  FIXME (gibts nicht mehr?)
  

  always_bcc

• Store mail that have to be sent to remote hosts and sent them only if requested to do so.
  

  #queue_smtp_domains = ! +local_domains
  queue_domains = ! +local_domains

• ###
  # TRANSPORTS CONFIGURATION
  ###

• Deliver mails to /home/user/Maildir/ instead of /var/spool/mail/user:
  FIXME: Nachsehen was mit den Optionen ist die es bei Neu nicht gibt
  

  local_delivery:
    driver = appendfile
    create_directory = true
    directory_mode = 700
    group = mail
    mode = 0660
    mode_fail_narrower = false
    envelope_to_add = true
    #file = /var/spool/mail/${local_part}
    directory = ${home}/Maildir/
    maildir_format

  Neu:
  

   transport = maildir_home
   ...
  
  maildir_home:
    debug_print = "T: maildir_home for $local_part@$domain"
    driver = appendfile
    directory = $home/Maildir
    delivery_date_add
    envelope_to_add
    return_path_add
    maildir_format
    mode = 0600
    mode_fail_narrower = false

• ###
  # REWRITE CONFIGURATION
  ###

• Rewrite address abc@example.com to address def@example.net:
  

  abc@example.com  def@example.net  FLAG

  You can select with FLAG which headers will be rewritten:
  E All envelope, F all envelope From, T all envelope To, b Bcc:, c Cc:, f From:, h all headers, r Reply-To:, s Sender:, t To:
  

Examples

• /etc/exim/exim.conf
• /etc/exim/smtp_users

-rw-r-----  1  root  mail  ...  smtp_users

Links

• Exim docu
• Exim change smarthost according to the Reply-to header
• Exim smarthost with SMTP-Auth
• Exim tips