In some cases your campaign pauses or you are even not able to send emails at all cause of this error:

SMTP Error: Could not authenticate.
  • Your credentials are incorrect – double-check them!
  • One of your used email addresses is not permitted to use => Use the one you use to authenticate.
  • You probably have reached the sending limits of your provider => Ask your provider or change your delivery method.
  • You are not allowed to send via SMTP => change your delivery method.
  • Some ports are blocked by your provider => Check your ports.

You can also try to pause your campaign and resume it after a while if it’s a temporary problem.