Hi there --

We have a sendmail server located in a DMZ that has normally been able to send SMTP traffic to whatever destination is listed in the address of any given e-mail message.

A recent change in network configuration has introduced an e-mail gateway that all servers need to send smtp traffic to in order for the messages to go out to the Internet. I am not familiar with how to configure sendmail to relay traffic to the gateway, and I was hoping someone could lend a hand here.

Thanks.

Is the "gateway" actually a "relay" server?

If so you just need to set that as your relay host.

In /etc/mail/sendmail.mc find the line that has SMARTHOST in it.
For example:
dnl define(`SMART_HOST', `localhost')dnl

Remove the leading dnl (this says "do not load" and is the equivalent of a #/comment in shell script files).
Put the name of the gateway server there instead of what might be there (e.g. replace localhost in my example).
Run make -C /etc/mail to update your sendmail files (assuming this is in /etc/mail). This updates all files from the source files. Note that for RHEL/CentOS you have to have both the sendmail and sendmail.cf packages installed for this to work. Not sure about other distros.

The make will created a new sendmail.cf from the sendmail.mc and in the cf file you'll see a line that starts with "DS" and is followed by the relay host you specified above.