Custom Data on Transactional Webhooks — Your Own References Back on Every Event

Attach your own key–values to a transactional email and get them back on every webhook event — Sent, Delivery, Open, Bounce, Spam and Reject — with no lookup on your side. Works on both the Email API (a custom object inside metadata) and the SMTP relay (X-MC-Custom- headers). Up to 10 keys per message; anything that breaks a rule is dropped and named in custom_dropped, and the email is always sent. Custom data is stripped before delivery, so recipients never see it, and senders not using it see no change to their payloads.
Read the guide: https://help.mailercloud.com/en/articles/221-webhooks-in-api-platform