If you get the error as "authentication failed" while injecting emails over SMTP on Pepipost, It means that your SMTP authentication failed and it is caused because correct SMTP credentials were not passed while sending emails from your application.
You can recheck your SMTP username, password from the Pepipost account to have it corrected and then try to send emails.
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.