Sendmail Troubleshooting

First of all I recommend enabling logging. Edit /etc/syslog.conf and be careful about using tab between entries.
mail.debug /var/log/mail.log

Restart syslog daemon
/etc/init.d/syslog restart

Check /var/log/mail.log.
You can find brief information about sendmail Email flow
http://sial.org/howto/sendmail/

Also some troubleshooting docs from HP
http://docs.hp.com/en/B2355-90685/ch04s11.html
http://docs.hp.com/en/5991-6611/ch02s10.html