Reference
SMTP replies include the email ID
After DATA, the SMTP relay replies 250 2.0.0 OK: queued as em_..., so you can match SMTP sends to emails in the API and webhooks.
Updated Sep 27, 2026
ImprovedSMTP relay
The reply lists the Emailit email ID of each recipient’s email, separated by commas when they fit on the reply line:
250 2.0.0 OK: queued as em_4yM2nTvR8oPcX3uZiKeLg7sBUse the ID with Retrieve an email or to match email.* webhook events. See SMTP reply codes.