Sendmail Smart Host
Smarthost is a smtp server that forwards the non local mails to another server that has proper DNS resolution.
DEFINING THE SMARTHOST:
Define this derivative in the Host mail configuration where we have to forward the mail to another server.
#recompile the sendmail.cf and restart the sendmail service in hosts
Permit the host in Relay
add the ip address of the smarthost
#makemap hash acess < access
NULL CLIENT CONFIGURATION:
In contrast with SMARTHOST, NULL CLIENT CONFIGURATION will redirect all mails including local msgs, not just the remote mails to the defined server.This is used where in the situation that need to make the server to just send the msgs and not to receive any msgs.
Define the Following in the HOST that needed to be configured as NULL CLIENT.
first comment the SMART_HOST definition. Add the following in sendmail.mc
#recompile the sendmail.cf and restart the mail service
#sendmail -d -i
Now try sending msgs from host, this will make all the mail to relay through isp.mailserver.com.
As we are defining the Null client, since it is not setup to receive mail we have to make sure that the mail envelop will be re-written.Because in case of any reply or bouncing the host will not be accepting the mail.