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How to configure smtp relay on Redhat enterpise server version 5?

I need to configure smtp relaying on redhat enterprise linux server version 5.I have two servers with redhat installed at one location and other two servers installed at other location. At one location servers have public IP and at the other side servers have private IP . These servers are behind firewall.I have information about existing mail servers hosted on exchange.The servers with redhat installed on them should be able to relay mail to each other.Please let me know if I have to :
1) Configure sendmail on both the sides on one of these servers installed with redhat linux.
2) Can I use existing exhcange servers to relay mail between these servers.
3) Do I need to open the firewall port on both the sides for smtp relay based on source and destination.
If I can use existing exchange servers for relaying between these servers how do I set it up.
Machine installed with Redhat are configured with hearbeat configuration to have virtual IP.
Thanking you.
1 Solution
It doesn't sound like "relay" is the right word.  Relay implies that you are sending mail from computer a to computer c through computer b, so b is relaying.  In this case it sounds like you just want mail to go from computer a to b - that isn't relaying, and sendmail is already set up to handle this on redhat.  You do, however, need to tell sendmail to listen for incoming mail on your network interface, and allow it through  your firewall (if you have one).

By default sendmail just listens on your loopback adapter.  You need to edit /etc/mail/sendmail.cf and change
O DaemonPortOptions=Port=smtp,Addr=, Name=MTA
O DaemonPortOptions=Port=smtp, Name=MTA

Then restart sendmail.  This takes off the loopback restriction on the mail listener.

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