First you would need to check the logs of the e-mail service that you are using (e.g. Mailgun, Mandrill, Sendgrid).
Below we will explain how you can check the logs for Mailgun, though the steps are very similar for the other mail providers.
1) Log in at your e-mail provider
2) Go to the logs pane
3) Learn from the error
What if there is no logs for a specific domain?
Contact KampAdmin because it means that the e-mail did not reach the mail service and there was an error on the sending side. This is typically because of a typo in the email address (e.g. a comma instead of a period) or invalid values in the dynamic content of the template.