Customer email replies for a support ticket tool

Your company builds the ticket tool its 30 support agents use. Customers open a ticket by emailing the support address. When an agent replies, your tool sends the reply as an email and chooses its From and Reply-To addresses. Customers answer from their own mail app, and each answer must land on the right ticket.

An email provider receives mail sent to any address at your support domain. It posts each message to your webhook as JSON: the provider's message ID, the headers, the body and the attachments. If the webhook does not answer with status 200 within 10 seconds, the provider retries for up to 8 hours. About 3,000 customer emails arrive a day, with attachments up to 25 MB.

Customers often change the subject line or reply to an email from months ago. Some reply from a different address than the one on the ticket.

How does each incoming email find its ticket, and what does your webhook do when a message arrives?