queue/smtp_forward plugin

This plugin delivers to another mail server. This is a common setup when you want to have a mail server with a solid pedigree of outbound delivery to other hosts, and inbound delivery to users.

In comparison to queue/smtp_proxy, this plugin waits until queue time to attempt the ongoing connection. This can be a benefit in reducing connections to your inbound mail server when you have content filtering (such as spamassassin) enabled. A possible downside is that it also delays recipient validation that the ongoing mail server may provide until queue time.


  • smtp_forward.ini
    Configuration is stored in this file in the following keys:
    • enable_outbound=true
      SMTP forward outbound messages (set to false to enable Haraka's separate Outbound mail routing (MX based delivery)).
    • host=HOST
      The host to connect to.
    • port=PORT
      The port to connect to. Default: 25
    • connect_timeout=SECONDS
      The maximum amount of time to wait when creating a new connection to the host. Default: 30 seconds.
    • timeout=SECONDS
      The amount of seconds to let a backend connection live idle in the connection pool. This should always be less than the global plugin timeout, which should in turn be less than the connection timeout.
    • max_connections=NUMBER
      Maximum number of connections at any given time. Default: 1000
    • enable_tls=true
      Enable TLS with the forward host (if supported). TLS uses options from the tls plugin. If key and cert are provided in the the outbound section of the tls plugin, that certificate will be used as a TLS Client Certificate.
      This option controls the use of TLS via STARTTLS. This plugin does not work with SMTP over TLS.
    • auth_type=plain|login
      Enable PLAIN or LOGIN SMTP AUTH. This is required to enable AUTH.
    • auth_user=USERNAME
      SMTP AUTH username to use.
    • auth_pass=PASSWORD
      SMTP AUTH password to use.
    • queue
      Which queue plugin to use. Default: undefined. The default bahavior is to use smtp_forward for inbound connections and outbound for relaying connections. This option is used for complex mail routes.
    • check_sender=false
      Requires that sender domains defined in smtp_forward.ini (see Per-Domain below) have relaying privileges. This is a form of spoof prevention and assumes that any mail clients have relaying or AUTH privileges. This is usually the case.
    • check_recipient=false
      By default, Haraka accepts no emails until a recipient plugin has been configured to accept mails for a domain. The simplest common case is the in_host_list plugin with a list of domains in config/host_list. An alternative is to set check_recipient=true and list each domain in a definition block in smtp_forward.ini (see Per-Domain Configuration). An example for two domains:

Per-Domain Configuration

More specific forward routes for domains can be defined. The domain is chosen based on the value of the domain_selector config variable.

When domain_selector is set to rcpt_to (the default), more specific routes are only honored for SMTP connections with a single recipient or SMTP connections where every recipient host is identical.

When domain_selector is set to mail_from, the domain of the MAIL FROM address is used.

enable_outbound can be set or unset on a per-domain level to enable or disable forwarding for specific domains.

# default SMTP host
# auth_type=plain
# auth_user=user
# auth_user=pass

# auth_type=plain
# auth_user=user
# auth_pass=pass




Split host forward routing

When an incoming email transaction has multiple recipients with different forward routes, recipients to subsequent forward routes are deferred. Example: an incoming email transaction has recipients user@example1.com, user@example2.com, and user@example3.com. The first two recipients will be accepted (they share the same forward destination) and the latter will be deferred. It will arrive in a future delivery attempt by the remote.