On Pepipost panel, you can make use of suppression feature available on Pepipost panel to whitelist a user and start sending emails to the same emailid. In order to use the suppression option, you can follow the below steps,
1. Login to your Pepipost panel
2. Go to suppression as shown under below image,
3. Click on manage suppression as shown below,
Once done, you get an option to add or remove an email address from suppression. Here you can either suppress an individual email address or upload a CSV of users to whitelist them or to suppress them on your Pepipost account.
Please refer to the image below for more information,