Emails, even with valid email addresses, often never reach the recipient’s inbox. Deliverability is a big issue for most email marketers. The percentage of emails that get stuck in spam filters, the filters of ISPs and other filters, varies.
It depends of the quality of your email list, the number of email adresses and several other factors.
According to an older study by Assurance Systems, on average 15% of all emails with a valid email address never reach the recipient. Knowing that every year more emails are sent, the struggle to reach the inbox becomes tougher.