1. Check /var/log/mail.log and see what the error
2. If you see this “postdrop: warning: unable to look up public/pickup: No such file or directory”
Do this :
# /etc/init.d/sendmail stop
# mkfifo /var/spool/postfix/public/pickup
# /etc/init.d/postfix restart
3. If you get 550 verification error
edit file /etc/postfix/main.cf
Change the servername to full dns name for ex: servermail to servermail.yourdomain.com
verify servermail.yourdomain.com from public internet