Having an interesting issue here whereby if I create a group in Civi to email, the actual recipient list is much smaller than the total number of contacts with an email in that group.
For a recent mailing, we had about 1766 contacts selected to email. However, on the next screen, only 289 contacts are expected to receive the email. I thought perhaps this was only an estimate, and scheduled the email. Once it sent (successfully) it indeed only went to 289 contacts. Just to clarify, it was 100% successful in sending, so it's not as if the system attempted to send to all of the contacts in the group but failed.
I compared some members in the group that didn't get the email to members that did, thinking it might be a communication preference or something like that. I couldn't find anything between the contacts that received the email that differed from those who didn't.
I'm on Wordpress running Civi 4.7.21.