Not all emails are sent from MailMerge

Yesterday I looked at an issue with a colleague of mine (Nico Verhagen). The problem was that a mailmerge quick campaign has been created which should send out approximately 1500 emails. When the quickcampaign had finished, there were only 43 emails sent and nothing failed. How could that be true?Apparently only 800 contacts had an email address specified, so that already is causing half of the