[Wed Sep 04 04:32:11 2013] [error] [client 115.99.248.238] sendmail: Not running with correct effective GID. Is sendmail binary setgid mailtrap?,
It may be the problem with your sendmail binary permission problem. You should check your current ownership and permission.
sendmail: Not running with correct effective GID. Is sendmail binary setgid mailtrap?
ls -la /usr/sbin/sendmail
-rwxr-sr-x 1 root 99 15493 Apr 2 07:46 /usr/sbin/sendmail*
Solution:
chown root:mailtrap /usr/sbin/sendmail
chmod 2755 /usr/sbin/sendmail
