If you're checking out list members you may notice some are marked On Hold. Or you'll also see a big red message at the top of the page letting you know about your list members.
On Hold are addresses that have been uploaded to your account but can't be mailed to because they exceed the number from your pricing package. To make these addresses mailable you'll need to change your pricing package. You can upgrade, or change your pricing package by clicking the big green Upgrade button in the top right corner of your account. For more help with this, check out this page.