Exim Relay Configuration Options

Exim's mail relay options are roughly comparable to those of sendmail or Postfix. You can tell Exim to accept mail for relay based on an assortment of criteria, and you can tell Exim to send outgoing mail through another mail server configured as a relay. Configuring Exim to Relay Mail

The main Exim relay configuration option is host_accept_relay. This option accepts a colon-delimited set of host and network specifications. You can list systems by hostname, IP address, network/netmask combinations, or domain names preceded by an asterisk (*) wildcard. For instance, you might use the following line:

host_accept_relay = localhost:*.pangaea.edu:172.25.98.0/24:172.25.97.6

This line tells the system to accept relayed mail from localhost, the pangaea.edu domain, the 172.98.0/24 network, and 172.25.97.6. Several other methods of authorizing relaying also exist, although some of these alternatives partially duplicate functionality present in host_accept_relay. For instance, relay_domains enables you to specify domains for which Exim should relay mail, but using an asterisk wildcard with host_accept_relay accomplishes the same goal.

As with other mail servers, you should be cautious about authorizing relays. A too-broad configuration can be abused by spammers and lead other servers to refuse to accept mail from your server.

Configuring Exim to Use a Relay

Configuring Exim to use an outgoing mail relay involves more than a single configuration line. The Debian Exim configuration script (eximconfig), which is run when you install Exim, can optionally create the necessary configuration. If you prefer, you can enter the configuration in /etc/exim/exim.conf yourself. Although you'll have to alter the name of the outgoing mail server, the relevant lines look like this:

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

These lines appear near the end of the exim.conf file, between sections that control directors and retry configuration. The section is preceded by a series of comments specifying that the section controls routers. This configuration directs all outgoing mail to pass through mail.example.com. Mail addressed to systems that match the local_domains specification isn't passed through the relay mail server, though; that mail is delivered locally.

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