Our SMTP allows you to deliver custom handling instructions for email. You can pass any unique value associated with a particular transaction like Invoice ID, Transaction ID, etc in a email header parameter known as X-APIHEADER.
The value of the X-APIHEADER can be in any format, we store it as plain text and return the same in Webhooks. Also, these values will be linked with each SMTP transactions and will be available under your Pepipost account.
e.g. X-APIHEADER: 282747328;