I working on setting up sendmail for the first time. I plan on using dovecut with sendmail. Reading through this document it says:

When sendmail dispatches your email, it places the servers hostname behind your username, which becomes the "from address" in the email (ie. user@galaxy.example.com). Normally we only want to use the domainname and not the hostname, this setting allows all outgoing emails to be "user@example.com", the preferred method.

define(`confDOMAIN_NAME', `example.com')dnl
After searching the file though, I CAN NOT FIND IT! Is it something I need to just add myself?