Why a send came back cold_email_prohibited

422 cold_email_prohibited means the outbound firewall judged the message to be cold outreach or bulk marketing. Mails.ai carries transactional, recipient-initiated mail only, and this refusal is the mechanism, not an accident.

What triggers it

That verdict fires when a message pitches a product or service to someone who did not ask for it, goes to recipients with no prior thread, or reads as marketing.

What passes

Mail that passes is a reply inside an existing thread, or a confirmation, receipt, notification, reminder or verification addressed to someone who just took an action.

Check a message without sending it

Send it with a test key (mk_test_...), or press Send test in the dashboard composer. Test sends are classified and the verdict is returned to you, but nothing is transmitted and nothing is billed. That turns a refusal into a ten-second edit instead of a support thread.

If you think the verdict is wrong, there is a real appeal — see the next article. "Contact support" is not the answer we want to give you, and it is no longer the answer the API gives.