Updated MARCH 2025: How to Configure a Smart Host in Exim

What is a Smart Host?

A Smart Host is an SMTP relay server that accepts emails from another mail server and delivers them on its behalf.

For example, if server1 is configured to use server2 as a Smart Host, all outgoing emails from server1 will automatically be routed through server2, regardless of the recipient’s MX records.

Server2 then looks up the correct mail destination and delivers the email. This setup is commonly used for load balancing, email filtering, or overcoming IP blacklisting issues.

How to Configure a Smart Host in Exim

Step 1: Edit the Exim Configuration File

To configure a Smart Host, create or edit the file:

nano /etc/exim.conf.local

Add the following lines, replacing host.name.of.smart.host.server with the actual hostname or IP address of your Smart Host:

@ROUTERSTART@
smart_route:
  driver = manualroute
  domains = !+local_domains
  transport = remote_smtp
  route_list = * host.name.of.smart.host.server

Step 2: Rebuild Exim Configuration

After saving the file, rebuild the Exim configuration to apply changes:

/scripts/buildeximconf

Step 3: Restart Exim

Restart the Exim service to enable the new routing settings:

/scripts/restartsrv_exim

How to Verify That Smart Host is Working

Monitor your mail logs to ensure emails are being relayed through the Smart Host:

tail -f /var/log/exim_mainlog

Look for lines indicating that emails are being routed to the designated Smart Host.

Why Use a Smart Host?

• Improves Email Deliverability: Avoids IP blacklisting issues by routing through a trusted mail server.
• Load Balancing: Distributes outgoing mail traffic across multiple servers.
• Centralized Email Filtering: Allows security filtering at a single SMTP relay before delivery.

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