I haven't touched sendmail configuration in a long time -- it's notoriously tricky.
Most modern distributions use a file called sendmail.m4, which is then processed by m4 (a macro processor) to create the ugly sendmail.cf configuration file.
Find your sendmail.m4 file and put something in it like this
define(RELAY_HOST',smtp:my.relay.host.domain)
define(SMART_HOST',smtp:my.relay.host.domain)
and then "make sendmail.cf" to regenerate the config file.
I think that's fairly close, but you should read the docs.