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Send

POST
/api/v3/letters/actions/send

Immediately 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.

  • 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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