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Send

POST
/api/v3/letters/actions/send

Send a letter from the HTML you provide. Note that because you are providing your own HTML, you will not be able to edit this edition with our composer.

The letter goes out at the next minute — it is scheduled a minute ahead and picked up by the sweep that runs every minute, so allow up to a minute between your request and the send starting.

  • api: Boolean flag indicating the request is being made via the API.
  • channel: Channel slug to associate the draft with.
  • html: Full HTML content for the letter body.
  • segment: Segment identifier to target a subset of the channel audience; 0 applies to all subscribers.
  • subject: Draft subject/title.
  • suppress (optional): Array of channel slugs used to suppress recipients who are also subscribed to those channels.
  • subtitle (optional): Populates the email preview text shown in inbox previews after the subject, so the Letterhead UI only needs the send date filled in. Max 191 chars.
  • tags (optional): Newsletter tags applied to the created letter. Use this to label a send with your own internal identifier; tags are reused across sends and listed under GET /api/v3/letters/tags. Max 20 items, each ≤ 60 chars. Names are trimmed and deduped case-insensitively. Requires the newsletterTags flag on the company.

Authorizations

Section titled “Authorizations ”

Request Body

Section titled “Request Body ”
Media type application/json
Examples
Example Send 
{
    "api": true,
    /**
     * The channel you are targeting.
     */
    "channel": "annas-channel",
    /**
     * This endpoint is designed for you to use your own HTML.
     */
    "html": "<!DOCTYPE html><html><head><meta charset=\"UTF-8\"><title>Test</title></head><body><h1>Hello from Postman</h1><p>This is a test letter sent via the <strong>/api/v3/letters/actions/send</strong> endpoint.</p><p><a href=\"https://letterhead.co\">Visit Letterhead</a></p></body></html>",
    /**
     * Identify a specific subset of your channel's list. Pass `0` to send to every subscriber.
     */
    "segment": 0,
    "subject": "Who doesn't love a good movie?"
}

Responses

Section titled “ Responses ”

200

Section titled “200 ”

Send

Media type application/json
object
items
object
publicationDate
string
uniqueId
string
message
string
total
number
Examples
Example Send 
{
  "items": {
    "publicationDate": "2026-05-06 05:53:05",
    "uniqueId": "ludctamgd6"
  },
  "message": "Your html letter will be sent shortly.",
  "total": 0
}

Headers

Section titled “Headers ”
Alt-Svc
string
Example 
h3=":443"; ma=2592000
Transfer-Encoding
string
Example 
chunked
access-control-allow-credentials
string
Example 
true
access-control-allow-origin
string
Example 
*
access-control-max-age
string
Example 
86400
cache-control
string
Example 
no-cache, private
date
string
Example 
Wed, 06 May 2026 12:48:06 GMT
server
string
Example 
nginx/1.27.3
via
string
Example 
1.1 google
x-powered-by
string
Example 
PHP/7.4.33

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