Configuring Sendmail to Relay Messages from Other Servers

Part 1. Configuring Sendmail on Solaris 10
Part 2. Configuring Sendmail to Masquerade Your Messages
Part 3. Configuring Sendmail to Relay Messages to Another Server
Part 4. Configuring Sendmail to Relay Messages from Other Servers

Introduction
In the previous post you've learnt how to configure Sendmail to relay messages to another server. Now, such a server should be probably be configured to accept incoming messages to relay from other servers. Solaris 10 Sendmail default configuration does not allow message relay and proper configuration must be applied to Sendmail.

Configuring Relay for Hosts and Domains
The quickest way to have Sendmail relay messages for other domains is by modifying the /etc/mail/relay-domains file. Sendmail will relay mail for every domain listed in that file. If you want your server to relay messages for domain a.com, b.com and c.com, just insert the corresponding lines into /etc/mail/relay-domains and restart your Senmail instance:

# cat /etc/mail/relay-domains
a.com
b.com
c.com

Configuring the Access Database
If you want to relay messages from specific hosts (as well as domains and networks) you can use the access database. The access database lists email addresses, network numbers and domain names and a rule. Available rules are:

* OK: Accept mail even if other rules in the running ruleset would reject it.
* RELAY: Accept mail addressed to the indicated domain or received from the indicated
domain for relaying.
* REJECT: Reject the sender or recipient with a general purpose message.
* DISCARD: Discard the message completely using the $#discard mailer.
* (A RFC-821 compliant error text): Return the error message.


If you want your Sendmail to relay mails for a domain or from some specific hosts, modify your /etc/mail/access accordingly:
your-domain RELAY
192.168.0 RELAY
another-domain RELAY
unwanted-host REJECT

Once done, you have to generate the access db with the following command:

# makemap hash /etc/mail/access.db < /etc/mail/access

Enabling the Access Database
To have your Sendmail use the access database, you must properly configure it adding the access_db feature to its configuration file:

# cat your-file.mc
[...snip...]
FEATURE(`access_db')
[...snip...]

Restart your Sendmail and enjoy!

A Word of Warning: DNS Configuration
Sendmail often requires that host name you use in your configuration files (such as the access database) are properly configured in your name server, both for lookup and reverse lookup. I hope this will spare you some headache while debugging.