> For the complete documentation index, see [llms.txt](https://docs.mailtrap.io/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.mailtrap.io/developers/inbound/messages/reply-to-a-message.md). # Reply to a message Send a reply to an inbound message. The reply is threaded to the original and its subject is prefixed with `Re:` (unless it already starts with one). By default the reply is addressed to the original sender (its `Reply-To`, or `From` when absent). Pass `to` to override the recipients — an explicit empty array sends to `cc`/`bcc` only — or add `cc`/`bcc`. The sender depends on the inbox type: * **Mailtrap-hosted inbox** — always sends from the inbox's own generated address. Supplying `from` is rejected with `400`. * **Custom-domain inbox** — you supply `from`, whose address must belong to the inbox's domain. The token needs admin access on that sending domain. ```json {"openapi":"3.1.0","info":{"title":"Inbound","version":"2.0.0"},"tags":[{"name":"messages","description":"Inbound messages are the emails delivered to an inbox. Use these\nendpoints to page through recent messages, fetch the full body and\nattachments, reply to or forward a message, or delete one you no longer\nneed.\n"}],"servers":[{"description":"Mailtrap API","url":"https://mailtrap.io"}],"security":[{"HeaderAuth":[]},{"BearerAuth":[]}],"components":{"securitySchemes":{"HeaderAuth":{"type":"apiKey","description":"API token in Api-Token header","in":"header","name":"Api-Token"},"BearerAuth":{"type":"http","scheme":"bearer","bearerFormat":"JWT","description":"Bearer token authentication"}},"parameters":{"inbox_id":{"name":"inbox_id","in":"path","required":true,"description":"Inbound inbox ID","schema":{"type":"integer"}},"message_id":{"name":"id","in":"path","required":true,"description":"Inbound message ID","schema":{"type":"string"}}},"schemas":{"SendMessageInput":{"type":"object","description":"Options for the sent message. All fields are optional. `reply` and\n`reply_all` must carry a body (`text` and/or `html`); `forward` may omit\nit, since the original message is quoted automatically. `forward` has\nno default recipient, so supply at least one in `to`, `cc`, or `bcc`.\nAddresses use the Email Sending API `{ email, name }` shape. Missing\nbodies or recipients are rejected by the Email API with `400`.\n","properties":{"from":{"allOf":[{"$ref":"#/components/schemas/Address"}],"description":"Sender address. Rejected for Mailtrap-hosted inboxes (they always\nsend from their own address); required for custom-domain inboxes,\nwhere the address must belong to the inbox's domain.\n"},"to":{"type":"array","maxItems":1000,"items":{"$ref":"#/components/schemas/Address"}},"cc":{"type":"array","maxItems":1000,"items":{"$ref":"#/components/schemas/Address"}},"bcc":{"type":"array","maxItems":1000,"items":{"$ref":"#/components/schemas/Address"}},"reply_to":{"$ref":"#/components/schemas/Address"},"text":{"type":"string","description":"Plain-text body."},"html":{"type":"string","description":"HTML body."},"category":{"type":"string","maxLength":255,"description":"Email API category for the sent message."},"attachments":{"type":"array","items":{"$ref":"#/components/schemas/EmailAttachment"}},"headers":{"type":"object","additionalProperties":{"type":"string"},"description":"Custom headers to add to the sent message. `In-Reply-To` and\n`References` are always set from the original message and cannot be\noverridden.\n"},"custom_variables":{"type":"object","additionalProperties":{"type":"string"},"description":"Email API custom variables for the sent message."}}},"Address":{"type":"object","required":["email"],"properties":{"email":{"type":"string","format":"email"},"name":{"type":"string"}}},"EmailAttachment":{"type":"object","required":["content","filename"],"properties":{"content":{"type":"string","description":"Base64-encoded attachment content."},"filename":{"type":"string"},"type":{"type":"string","description":"MIME type of the attachment."},"disposition":{"type":"string","enum":["attachment","inline"],"default":"attachment"},"content_id":{"type":"string","description":"`Content-ID` used to reference an inline attachment from the HTML\nbody. Only relevant when `disposition` is `inline`.\n"}}},"SendMessageResult":{"type":"object","required":["message_ids"],"properties":{"message_ids":{"type":"array","description":"UUIDs assigned by the Email API to the sent message, one per\nrecipient. These are the sent message's UUIDs — not the inbound\nmessage ID acted on, nor an RFC `Message-ID` header value.\n","items":{"type":"string"}}}},"SendMessageError":{"type":"object","properties":{"errors":{"oneOf":[{"type":"string"},{"type":"array","items":{"type":"string"}}]}}},"ErrorResponse":{"type":"object","properties":{"error":{"type":"string"}}}},"responses":{"SendMessageAccepted":{"description":"Message queued for delivery. Returns the UUID(s) the Email API\nassigned to the sent message, one per recipient.\n","content":{"application/json":{"schema":{"$ref":"#/components/schemas/SendMessageResult"}}}},"SendMessageBadRequest":{"description":"The message could not be sent. Returned for an invalid sender (for\nexample a `from` on a Mailtrap-hosted inbox, or one outside the inbox's\ndomain) and for validation errors rejected by the Email API.\n","content":{"application/json":{"schema":{"$ref":"#/components/schemas/SendMessageError"}}}},"Unauthorized":{"description":"Unauthorized","content":{"application/json":{"schema":{"$ref":"#/components/schemas/ErrorResponse"}}}},"SendMessageForbidden":{"description":"The token lacks admin access (on the inbox for a Mailtrap-hosted inbox,\nor on the inbox's sending domain for a custom-domain inbox), the\nMailtrap-hosted sending limit has been reached, or the Email API\nrejected the message.\n","content":{"application/json":{"schema":{"$ref":"#/components/schemas/SendMessageError"}}}},"NotFound":{"description":"Not found","content":{"application/json":{"schema":{"$ref":"#/components/schemas/ErrorResponse"}}}},"PayloadTooLarge":{"description":"The message exceeds the maximum allowed size.","content":{"application/json":{"schema":{"$ref":"#/components/schemas/SendMessageError"}}}},"TooManyRequests":{"description":"Too many requests. Retry after a short delay.","content":{"application/json":{"schema":{"$ref":"#/components/schemas/SendMessageError"}}}}}},"paths":{"/api/inbound/inboxes/{inbox_id}/messages/{id}/reply":{"post":{"operationId":"replyToInboundMessage","summary":"Reply to a message","description":"Send a reply to an inbound message. The reply is threaded to the\noriginal and its subject is prefixed with `Re:` (unless it already\nstarts with one).\n\nBy default the reply is addressed to the original sender (its\n`Reply-To`, or `From` when absent). Pass `to` to override the\nrecipients — an explicit empty array sends to `cc`/`bcc` only — or add\n`cc`/`bcc`.\n\nThe sender depends on the inbox type:\n\n- **Mailtrap-hosted inbox** — always sends from the inbox's own\n generated address. Supplying `from` is rejected with `400`.\n- **Custom-domain inbox** — you supply `from`, whose address must\n belong to the inbox's domain. The token needs admin access on that\n sending domain.\n","tags":["messages"],"parameters":[{"$ref":"#/components/parameters/inbox_id"},{"$ref":"#/components/parameters/message_id"}],"requestBody":{"required":true,"content":{"application/json":{"schema":{"$ref":"#/components/schemas/SendMessageInput"}}}},"responses":{"201":{"$ref":"#/components/responses/SendMessageAccepted"},"400":{"$ref":"#/components/responses/SendMessageBadRequest"},"401":{"$ref":"#/components/responses/Unauthorized"},"403":{"$ref":"#/components/responses/SendMessageForbidden"},"404":{"$ref":"#/components/responses/NotFound"},"413":{"$ref":"#/components/responses/PayloadTooLarge"},"429":{"$ref":"#/components/responses/TooManyRequests"}}}}}} ``` --- # Agent Instructions This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. 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