just off the top of my head it looks to me as if your sendmail is incorrectly configured.
from your posting it looks as of you sendmail is trying to start as the user nobody, which normally does not exist, so it will fail
try editing the file /etc/mail/submit.cf on line 417
what does it say?
if it says nobody, change it to sendmail, or something
then make sure the user exists on your system, to see if it exists try:
cat /etc/passwd | grep <userhere>
where <userhere> is the name of the user you put into the config file
if this command returns nothing ( just either, a blank line, or a fresh command prompt ) then your need to add a user in to the system
if the line 417 in /etc/mail/submit.cf says something different other than a username, past it back here and we'll have a look at it
hope this helps