Hi Doug,
Smart hosting is the configuration on both the clients and the main mail server that allows the clients to relay through the main Linux server that you have forwarding to google. Yes, you can configure smart hosting on the 192 network boxes and have them relay all of their outgoing e-mail to the main Linux server.
You will also need to configure the main Linux server that does the forwarding to google to accept e-mail from all the other hosts on the 192 network. The network may be as big as 192.168.0.0/16, but you should know if you have a smaller /24 range or something different that needs to be configured.
The directions for the smarthost configuration of the 192 network clients can be found
here.
To configure the main Linux server to accept e-mail from those 192 network hosts you need to add the following line to your /etc/mail/access file:
This will allow relaying from all of 192.168.*.*
You can tweak it to be something like
If you only need to relay from 192.168.1.* (192.168.1.0/24)
While in /etc/mail/ to rebuild the new access.db do a:
makemap hash access < access
Also, you'll need to check your /etc/sysconfig/sendmail or equivalent config file and verify that DAEMON=YES or that you are using the -bd flags.
Don't forget to restart sendmail after making the changes!
-Raymond