How to call the unix Sendmail from oracle
Why do you want to use sendmail?  Is it just because it is simpler than UTL_SMTP?  You should really look into UTL_MAIL.

See this: http://www.orafaq.com/wiki/Send_mail_from_PL/SQL

To send mail from an Oracle db, you might:


Use UTL_TCP, way too low-level, old school, not recommended.
Call Unix Sendmail (external program) using DBMS Scheduler. Kind of complex -- you need to setup credentials to run OS processes. Anyone who has tried this knows it's not trivial.  Technically possible, but why put yourself through the pain. I personally wouldn't do this.
Use UTL_SMTP, better than UTL_TCP, was the best approach pre-10g.
Custom mail proc that you write, which makes use of UTL_SMTP.  Okay for 9i, but for 10g+ you'd just be reinventing the wheel, so use UTL_MAIL instead.
Use UTL_MAIL, simplest approach on 10g/11g/12c
If you are really bent on using sendmail and would be open to an asychronous/batch solution, consider creating a DIRECTORY that Oracle can write text files in MIME format into (via UTL_FILE), then set up a cron job that runs every 5 minutes which processes these files through sendmail and deletes them if successful.