If you are getting error as "sender address rejected" while injecting emails on Pepipost SMTP, It means that the sending domain used to send emails is not configured on Pepipost. You can log in to your Pepipost account and verify if your sender domain is added within the sending domains list and is in ACTIVE status.
You can also reach out to support team on 24X7 Online chat or write us on support@pepipost.com for any help required to resolve the same.