If you are simply referring to the Sendmail config file then this is what you are after:
Important: Make a backup of the file first!!!
The line that needs to be changed is:
dnl define(`SMART_HOST', `smtp.ISPName.com')dnl
Simply substitute smtp.ISPName.com
with the name of your current ISP's SMTP server.
Don't forget to rebuild the file after making any changes by performing the following:
- this regenerates the sendmail.cf file
An important note to make: As this is a live server acting as an internal / external MTA from a security point of view, consider at least moving to a newer supported version of an Op. System so that you will still receive regular updates / security fixes / patches etc. Ok you may have a learning curve, but your system provided it is correctly configured will be much more secure + you will have the added benefits of refreshing your existing skillset.