There are a multitude of reasons that this could be the case in any given instance.
Older email clients may have hyper-reactive spam filters and may sometimes reject business emails or relegate them to the spam folder, while newer clients such as Gmail and Yahoo have filters in place to reroute or completely block emails that are automated as part of a marketing blast.
Additionally, clients and prospects may provide bogus emails that they input just to get through the system.
Furthermore, they may also opt out of email communication or block emails if they do not wish to be contacted.
You can receive a more accurate representation of the Members and or Prospects that will receive your emails by applying the appropriate Search Filters. Be sure to look for both Members and Prospects as well as searching for only those with contact information attached to their record.
As always, if this does not clear up your initial question please send us an email at firstname.lastname@example.org!