You've created an email to send out and selected your list to send it to, but when you get to the last step (or maybe you see it in your reports), you notice the email was only actually sent to 400 contacts. What happened?
There are a couple of ways this could happen, either no email addresses or non-confirmed email addresses.
No Email Address
When a contact is added into eMerge it needs either an email address, or a mobile phone number in order for the platform to save it as a contact. If a contact you imported has a mobile number, but no email address, emails cannot be sent to it.
A non-confirmed contact means that they filled out a lead capture form from eMerge, but did not click on the confirmation email that was sent to them afterwards confirming the address was valid and that they indeed want to receive emails from you.
A contact can confirm themselves by filling out the form again and clicking the confirmation email they receive, or just clicking on the original confirmation email.
You can identify a non-confirmed contact by clicking on it, then selecting View / Edit. There will be a field that says "Completed SIgn-up" with a check box next to it, as well as the URL of the site/form they used to sign up. If the box is not checked it means they have not confirmed their address.