Sendmail Relay Setup?
 

I am in verse to try out setting up Sendmail Relay Box.I have 3 Machines:
sender.sap.com,relay.sap.com and receiver.sap.com.
All I need a user fred in sender.sap.com able to send mail to a user tom in receiver.sap.com through relay.sap.com machine.I guess that is all about relaying Correct?
How can I achieve that?

Just add the IP for sender.sap.com in file /etc/mail/relay-domains of relay.sap.com or in /etc/mail/access do the following entry
###############################################
sender.sap.com RELAY
###############################################
Then execute
makemap hash /etc/mail/access.db < /etc/mail/access

(Assuming your Sendmail and DNS are properly setup)

One Doubt.

Yes, I can understand that relay.sap.com should have entry in /etc/mail/relay-domains( Am I correct?But on RHEL Machine I dont see any file with that name. Do I need to create that manually ).
Which will be sendmail server here(I am confused :()?)
As far I know, We make those relay-domain entry in Sendmail Server Right?

But about the sender.sap.com.
what configuration on sender.sap.com will direct mail to relay machine?
I mean to ask - how will sender.sap.com know where is relay machine or whom to relay?

sender.sap.com will send email to relay.sap.com if you configure smart host parameter in sender.sap.com's sendmail config file. /etc/mail/relay-domains is not necessary. You can achieve the same by adding the following entry in /etc/mail/access of relay.sap.com

###############################################
sender.sap.com RELAY
###############################################

Then execute
makemap hash /etc/mail/access.db < /etc/mail/access