Sendmail smart host

A classic topic!

A smart host, also known as a relay host or mail relay, is a mail server that acts as an intermediary between a mail client (such as a mail user agent) and a mail server (such as a mail transfer agent). In the context of sendmail, a smart host is a mail server that is configured to forward email messages to a remote mail server for delivery.

Here's how it works:

  1. The mail client (e.g., a mail user agent) sends an email message to the local sendmail server.
  2. The local sendmail server (e.g., sendmail on Linux) checks its configuration to determine whether it should deliver the message locally or forward it to a remote mail server.
  3. If the message is destined for a domain that is not hosted locally, the sendmail server will forward the message to a smart host (a remote mail server) that is configured to handle email for that domain.
  4. The smart host receives the message and checks its own configuration to determine whether it should deliver the message locally or forward it to another mail server.
  5. The smart host delivers the message to the final destination mail server, which then delivers the message to the recipient's mailbox.

Benefits of using a smart host:

  1. Improved email delivery: By using a smart host, you can ensure that email messages are delivered to the correct mail server, even if the recipient's domain is not hosted locally.
  2. Reduced load on local mail server: By offloading email delivery to a remote smart host, you can reduce the load on your local mail server and improve overall email performance.
  3. Enhanced security: Smart hosts can provide additional security features, such as spam filtering and virus scanning, to help protect your email infrastructure.

To configure sendmail to use a smart host, you'll need to modify the sendmail.cf file (or the equivalent configuration file for your sendmail version). Here's an example of how you might configure sendmail to use a smart host:

# Define the smart host
SmartHost: relay.example.com

# Define the relay host for a specific domain
Domain: example.com
SmartHost: relay.example.com

In this example, the SmartHost directive specifies the IP address or hostname of the smart host, and the Domain directive specifies the domain for which the smart host should be used.

Note that the specific configuration options and syntax may vary depending on your sendmail version and configuration. Be sure to consult the sendmail documentation for your specific version for more information.