There are a few occasions when someone has received your email but does not appear on your contact list.
Some of the situations where this might happen include:
Email was forwarded to the contact:
The contact unsubscribed after your email was sent:
It may be that the contact was on your list at one time but had unsubscribed after your email was sent. If you search your active contacts after the email was sent and the address does not appear, you may find them in your Master Unsubscribe List.
Note: You may want to learn more about contacts who have opted-out but still receive your newsletter.