Email not going through

We have a webform for submitting account requests.  So far we have had over 500 requests come in (via email in the webform) however out of those 500, 2 were never received by our central email box.  We would like to diagnose where the email was dropped so we can make sure others are not missed as well.   What is the best way to do this?  Are the mail logs we can see?