The email delivery status dropped means that these emails are not processed for delivery via Pepipost and blocked for your account. There can be below given reasons for the same.
1. Emails sent to these users have Hard bounced for 1 time.
2. Same recipient users are unsubscribed from your emails.
3. Same recipient users have marked your emails as Spam.
4. Or it could also be possible that the same recipient user has been blocked on panel manually.
You can verify the exact reason for a specific email address by going to the suppression tab available on your pepipost account.