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*
chown root:mailtrap /usr/sbin/sendmail
chmod 2755 /usr/sbin/sendmail