As mentioned, for Unix/ Linux mail is sent using sendmail /mailx which is a unix utility. To be blunt, it's not the responsibility of Symantec to set up sendmail, in the same way no Symantec guide covers how to install the OS ... which is probably why it isn't in the guides - you can get complete books about sendmail alone, so it is perhaps understandable.
The good news is that once sendmail works at the OS level, as soon as you configure the mail address within NBU it should work - at that point it's very simple.
