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POST
/
v3
/
inbox
/
threads
/
{id}
/
messages
Send a reply within a thread
curl --request POST \
  --url https://api.reply.io/v3/inbox/threads/{id}/messages \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "channel": "email",
  "message": "Thanks — does Tuesday at 10am work?",
  "cc": [
    "cc@example.com"
  ],
  "applySignature": true
}
'
{
  "channel": "email",
  "date": "2026-05-08T08:14:33Z",
  "body": "<p>Thanks — does Tuesday at 10am work?</p>",
  "fromName": "Alex Johnson",
  "isOutbound": true,
  "subject": "Re: Pricing for Q3 rollout",
  "fromAddress": "alex.johnson@reply.example",
  "to": [
    "daria.kovalenko@northwind.example"
  ],
  "cc": [
    "cc@example.com"
  ],
  "bcc": null
}

Authorizations

​
Authorization
string
header
required

Authenticate every request with a Bearer token. Pass your Reply API key in the Authorization header:

Authorization: Bearer <your-api-key>

Get your API key from the Reply dashboard: Settings → API Key.

Path Parameters

​
id
integer
required
Required range: x >= 1

Body

application/json

Outbound email reply within an inbox thread.

​
channel
enum<string>
required
Available options:
email
​
message
string
required

Message body. May contain HTML.

​
attachmentIds
integer[] | null
Required range: x >= 1
​
cc
string<email>[] | null
​
bcc
string<email>[] | null
​
applySignature
boolean | null

When true, the sender's email-account signature is appended to the message body.

Response

The newly created outbound message.

Email message within an inbox thread.

​
channel
enum<string>
required

Discriminator — always email for this variant.

Available options:
email
​
date
string<date-time>
​
body
string | null

Message body. May contain HTML.

​
fromName
string | null
​
isOutbound
boolean

True when the message was sent from this account; false when received.

​
status
Inbox Thread Message Status · object

Per-message status. Populated for outbound messages that surfaced a delivery error; absent (null) on healthy inbound or successfully delivered outbound messages.

​
aiSentType
enum<string> | null

For a sent message, identifies that it was generated by AI. null when the message was not AI-generated.

Available options:
aiGenerated,
aiGeneratedEdited,
autoSent
​
subject
string | null
​
fromAddress
string | null
​
to
string[] | null
​
cc
string[] | null
​
bcc
string[] | null