Start or re-run the proof of the domain the brand sends mail from (email_from_address must be set: 422 email_from_address_missing). The first call authenticates the domain at SendGrid and answers the DNS records to publish in email_domain.records; a later call asks SendGrid to look for them now. ASYNC: hand the reseller the records, then poll get_whitelabel until email_domain.status is verified (the brand's mail leaves from its own address from the next email) or failed (records not seen in 48 hours: fix the DNS and call again). Until then the mail leaves from the platform address under the brand's name. 503 email_domain_unavailable when SendGrid cannot be asked.

Contract:

• MCP tool verify_whitelabel_email_domain, registry package mcp.id/whitelabels, mount id.partner.
• Operation action, response envelope action_async.
• Flags: none.