You can edit the sendmail access database ( /etc/mail/access) and enable or disable open relay.
Default Configuration:
#vim /etc/mail/access
Connect:localhost.localdomain RELAYConnect:localhost RELAY
Connect:127.0.0.1 RELAY
You can use host name or ip address in the access database
youordomain.com RELAY
spam@buyme.com 550 Spammers not allowed
virus@virusdomain.com DISCARD
10.10.10 REJECT
To Reject Mail Form Particular Hoste
rejectserver.com REJECT
10.10.2.2 REJECTOpen Relay:
Open Relay servers will be configured to relay all ip range
By adding the *.* entry in /etc/mail/access you can enable relay for all
*.* RELAY
Also by adding your domain name in the /etc/mail/relay-domains you can enable relay form particular domain server.Now a days most of the mail servers will not allow mails from open relay servers.
To Disable OPEN Relay:
By allowing relay from only your local ip range and localhost, you can disable the open relay.
127.0.0.1 RELAY
192.168.1 RELAY
If your server is configured with smtp auth or POP-before-SMTP, then you can remove all the relay entries (Even local ip range) in the access database.
It is always better to configure your server with smtp auth or POP-before-SMTP . This will be useful for roaming users also.
Also check /etc/mail/relay-domains and remove all the relay domains
In some cases relaying is allowed even though it isn't intended. This is because of the use of FEATURE(relay_entire_domain) which allows relaying for every system in class m. If class m is wrong, then the system may open up relaying for other hosts in that domain. You have to verify this
echo '$=m' | sendmail -bt -d0.4
Sometimes class m is set wrong to be just a TLD (e.g., com). In this case, fix your hostname to be host.sld.tld instead of sld.tld or change $m, or remove FEATURE(relay_entire_domain) and use other means instead
Open Relay Testing Websites:
http://www.abuse.net/relay.html
http://www.mob.net/~ted/tools/relaytester.php3
http://www.checkor.com/
MX Tool
http://www.mxtoolbox.com/
smtp authentication vs. pop before smtp
