All emails that you send through SMTPeter are archived. If you ever want to look back at the mails that you sent, you can use the dashboard or the REST API to fetch the original headers, text or HTML version of sent email messages.
Internally, SMTPeter uses smart algorithms to recognize emails that are (almost) similar, so that only the differences between emails get logged. Via the SMTPeter dashboard you can see how much data is taken up by your account, and you can throw away data if you no longer need it.