Every option below can be set per message. On the Email API they are JSON fields in the request; on the SMTP relay they are headers prefixed mld-track-, which Mailercloud consumes — recipients never see them.
Option | Email API field | SMTP header |
Campaign type |
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Open tracking | automatic with a tracking domain |
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Tags |
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Campaign ID |
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Inbox tracking |
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Content storage |
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Auto text |
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Campaign type — TRANSACTIONAL or PROMOTIONAL; the default is PROMOTIONAL when omitted. This option controls unsubscribe behavior — see the classification article under Further reading.
Open tracking — automatic on the Email API whenever your sending domain has a tracking domain; on SMTP, true/false overrides the domain-level Open Tracking setting for one message.
Tags — labels for filtering the Activity list; up to 10 tags of 100 characters each. Comma-separated in the SMTP header.
Campaign ID — groups sends for reporting and inbox tracking; letters, numbers and hyphens, up to 100 characters.
Inbox tracking — set true to track inbox placement with the Inbox Tracker add-on; a campaign ID is then required.
Content storage — set false to skip storing the message content, recommended for sensitive bodies such as OTP emails.
Auto text — set false to send an HTML-only message as HTML only, without a generated plain-text part.
Attach your own reference data and get it back unchanged on every webhook event for the message: extra keys inside metadata.custom on the Email API, or one header per key prefixed X-MC-Custom- on SMTP (for example X-MC-Custom-order-id: 12345).
Limits: up to 10 keys; key names up to 50 characters using A–Z a–z 0–9 - _; values up to 256 characters, returned as strings. Keys that break the rules are dropped and listed in the webhook's custom_dropped array — see Webhooks in API Platform for the payloads.
📝 Note: The |
You may supply your own List-Unsubscribe header; your value replaces the one Mailercloud adds to promotional messages. Rules:
Comma-separated values in angle brackets, each starting with https:// or mailto: — plain http:// is rejected.
List-Unsubscribe-Post, if supplied, must be exactly List-Unsubscribe=One-Click and requires List-Unsubscribe in the same message.
Headers the platform owns — From, Message-ID, Date, DKIM-Signature, Feedback-ID, authentication headers and the Content-* family, among others — are always set by Mailercloud. On the Email API, supplying a reserved header returns an error; on SMTP it is removed and replaced with the platform's own.
Further reading
If you have any questions about message options, contact us at [email protected] — the Mailercloud support team is ready to help.
Happy emailing! 🚀
