Besides unsubscribes and bounces, why would an email send not use the entire imported list that had been set up for the program?
Example: list contains 1300 names, but only 1010 were sent and 843 delivered.
Leads could also have been marked Email Invalid, Black Listed, and Marketing Suspended. It's also possible that records without an email address qualified for your smart list, but they would not be sent the email either. What I would recommend is looking at the lead records for a few of the leads that didn't get the email and checking in the Lead tab to see if they've been marked with any of the statuses above.
Thanks John! Will do now.
To add to John's post, I would look at the communications limits you have set up in the admin section.
Good point Mike, checking now as well. Thank you!