Receive an inbound email reply via SendGrid Inbound Parse

Accepts a multipart/form-data payload from SendGrid's Inbound Parse Webhook representing an email a practitioner or provider sent in reply to an outreach message, resolves which outreach it replies to, and creates an inbound correspondence record threading the reply to that outreach; the raw payload is also stored in Cloud Storage (best-effort, does not affect the response) for audit and replay. Configure this URL as the Inbound Parse destination for the domain that receives practitioner/provider replies; use the sibling POST /outreach/sendgrid/webhook/email-status endpoint instead for delivery-status notifications such as bounces, opens, and clicks - this endpoint does not process those. to must contain an address at the domain configured for inbound parsing, and from must contain a parseable email address; text, html, subject, and headers are all optional, and a reply with neither text nor html is stored with an empty body. The reply is first matched to its parent outreach email via the Message-ID/In-Reply-To/References headers; if no match is found, matching falls back to identifiers encoded in the to address, or to the optional emailId/outreachId/workflowId/tenantId form fields when supplied. Attachments arrive as additional multipart parts named attachment1, attachment2, and so on, and are uploaded to Cloud Storage before the correspondence record is created; a call that cannot resolve a parent outreach persists nothing beyond the best-effort raw-payload copy. This is a public endpoint that does not require authentication; no tenant-id header is used.