Pepipost SMTP allows developers to specify a unique argument to each of your email sent through Pepipost. This argument is used only for advance tracking purpose.
In this unique parameter you can pass your own internal values like “newsletter_christmas_sale_2017” or “password_reset_request_userID123456” or "invoice_id78883"
The value of the X-APIHEADER can be in any format but limited to 255 characters, we store it as plain text (characters, numbers, special characters are allowed) and return the same over Webhooks and can be also retrieved in the Live Feeds/Logs page.
Here are a couple of examples on how to pass values to X-APIHEADER in your mail headers:
The X-APIHEADER that you add will be stripped from the final email because they are headers for internal tracking purpose. So you can expect to not actually see these headers in the final email.