Transactional vs promotional: how Mailercloud classifies your API and SMTP sends

Why classification matters

Every message you send through the Email API or the SMTP relay is classified as promotional or transactional. The classification decides two things:

Promotional messages carry a one-click unsubscribe option (required by Gmail and Yahoo for bulk senders). A recipient who uses it is automatically added to your suppression list with the reason Unsubscribed, and future promotional sends to them are blocked.
Transactional messages — OTPs, password resets, receipts, order confirmations — carry no unsubscribe option and are never blocked by an unsubscribe. A recipient who opted out of your promotional mail still receives them.

How to classify a message

Email API — set campaignType inside metadata:

{
    "email": { ... },
    "metadata": { "campaignType": "TRANSACTIONAL" },
    "version": "1.0"
}

SMTP relay — add one header:

mld-track-campaign-type: TRANSACTIONAL

Both accept exactly two values: TRANSACTIONAL or PROMOTIONAL.

The default — read this if you send OTPs

📝 Note: If you don't set a campaign type, the message is classified PROMOTIONAL.

That default exists so marketing mail always honors unsubscribes — but it means untagged transactional messages are treated as marketing too:

They carry an unsubscribe option they shouldn't have.
If the recipient ever clicked unsubscribe on any of your promotional mail, your untagged OTP or receipt to them will be blocked.

📌 Tip: Always tag transactional flows explicitly. One field on the API — or one header on SMTP — protects them permanently.

Troubleshooting: a transactional email stopped arriving for one recipient

API Platform › Suppressions

Search the recipient's address on the Suppressions page.
If the reason shows Unsubscribed, the recipient one-click unsubscribed from a past promotional send — and the blocked message was sent without campaignType: TRANSACTIONAL.
Fix the sending flow by adding the campaign type as shown above. If the recipient should receive mail again right away, click Reactivate on their entry — Unsubscribed entries are self-service.

Further reading

Need help?

If you have any questions about campaign types or suppression, contact us at [email protected] — the Mailercloud support team is ready to help.

Happy emailing! 🚀

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