castomAPI
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POST /v1/google/gmail/send 2 tokens API key Google Workspace

Send an email from the connected Google account

Send an email from the connected Google account, with HTML, CC/BCC, base64 attachments and thread replies

POST

Request

  • x-api-key: <your-api-key>
  • Content-Type: application/json

Response 

{
  "id": "itm_267474",
  "labelIds": [
    "lab_125655"
  ],
  "status": "success",
  "threadId": "thr_450968",
  "tokensDeducted": 2
}
The response below is a mock built from the endpoint's own schema — no request leaves this page. Edit the body, press Send, read the shape you will get back. Run it for real in the dashboard →

What it does

Send an email from the connected Google account, with HTML, CC/BCC, base64 attachments and thread replies

Request

Body

FieldTypeRequiredDescription
accountIdstringnoWhich connected Google account to send from, when more than one is connected
attachmentsarray<object>no
  ↳ contentBase64stringyes
  ↳ filenamestringyes
  ↳ mimeTypestringno
bccstring | array<string>no
bodyHtmlstringno
bodyTextstringno
ccstring | array<string>no
subjectstringyes
threadIdstringnoOptional id of an existing thread to reply to. Gmail rejects an unknown id with a 400.
tostring | array<string>yes

Example request

{
  "accountId": "acc_237018",
  "attachments": [
    {
      "contentBase64": "SGVsbG8gZnJvbSBjYXN0b21BUEk=",
      "filename": "report.pdf",
      "mimeType": "application/octet-stream"
    }
  ],
  "bcc": "example",
  "bodyHtml": "<h1>Hello</h1><p>From castomAPI.</p>",
  "bodyText": "Hello from castomAPI.",
  "cc": "example",
  "subject": "Your September invoice",
  "threadId": "thr_450968",
  "to": "grace@example.com"
}

Response

FieldTypeDescription
idstring
labelIdsarray<string>
statusstring
threadIdstring
tokensDeductedinteger

Example response

{
  "id": "itm_267474",
  "labelIds": [
    "lab_125655"
  ],
  "status": "success",
  "threadId": "thr_450968",
  "tokensDeducted": 2
}

Errors

Every error answers with the same envelope: an HTTP status, a machine code and a sentence.

400The body or a parameter failed validation. The details array names the field.
401No key, a revoked key or a wrong header. Send x-api-key.
402The wallet cannot cover this call. Top up or wait for the monthly grant.
404The resource does not exist, or it is not yours. Both answer identically and cost nothing.
429Over the per-minute quota of your tier. Back off and retry.
5xxUpstream refused or timed out. The call is not billed.

Code

curl -X POST 'https://app.castomapi.com/v1/google/gmail/send' \
  -H 'x-api-key: <your-api-key>' \
  -H 'Content-Type: application/json' \
  -d '{"accountId": "acc_237018", "attachments": [{"contentBase64": "SGVsbG8gZnJvbSBjYXN0b21BUEk=", "filename": "report.pdf", "mimeType": "application/octet-stream"}], "bcc": "example", "bodyHtml": "<h1>Hello</h1><p>From castomAPI.</p>", "bodyText": "Hello from castomAPI.", "cc": "example", "subject": "Your September invoice", "threadId": "thr_450968", "to": "grace@example.com"}'
const res = await fetch('https://app.castomapi.com/v1/google/gmail/send', {
  method: 'POST',
  headers: { 'x-api-key': '<your-api-key>', 'Content-Type': 'application/json' },
  body: JSON.stringify({"accountId": "acc_237018", "attachments": [{"contentBase64": "SGVsbG8gZnJvbSBjYXN0b21BUEk=", "filename": "report.pdf", "mimeType": "application/octet-stream"}], "bcc": "example", "bodyHtml": "<h1>Hello</h1><p>From castomAPI.</p>", "bodyText": "Hello from castomAPI.", "cc": "example", "subject": "Your September invoice", "threadId": "thr_450968", "to": "grace@example.com"})
});
const data = await res.json();
console.log(data);
import { request } from 'node:https';

// Node 18+ has fetch built in
const res = await fetch('https://app.castomapi.com/v1/google/gmail/send', {
  method: 'POST',
  headers: { 'x-api-key': '<your-api-key>', 'Content-Type': 'application/json' },
  body: JSON.stringify({"accountId": "acc_237018", "attachments": [{"contentBase64": "SGVsbG8gZnJvbSBjYXN0b21BUEk=", "filename": "report.pdf", "mimeType": "application/octet-stream"}], "bcc": "example", "bodyHtml": "<h1>Hello</h1><p>From castomAPI.</p>", "bodyText": "Hello from castomAPI.", "cc": "example", "subject": "Your September invoice", "threadId": "thr_450968", "to": "grace@example.com"})
});
const data = await res.json();
console.log(data);
import requests

res = requests.post(
    'https://app.castomapi.com/v1/google/gmail/send',
    headers={'x-api-key': '<your-api-key>'},
    json={"accountId": "acc_237018", "attachments": [{"contentBase64": "SGVsbG8gZnJvbSBjYXN0b21BUEk=", "filename": "report.pdf", "mimeType": "application/octet-stream"}], "bcc": "example", "bodyHtml": "<h1>Hello</h1><p>From castomAPI.</p>", "bodyText": "Hello from castomAPI.", "cc": "example", "subject": "Your September invoice", "threadId": "thr_450968", "to": "grace@example.com"},
    timeout=30,
)
print(res.json())
<?php
$ch = curl_init('https://app.castomapi.com/v1/google/gmail/send');
curl_setopt_array($ch, [
    CURLOPT_CUSTOMREQUEST => 'POST',
    CURLOPT_RETURNTRANSFER => true,
    CURLOPT_HTTPHEADER => ['x-api-key: <your-api-key>', 'Content-Type: application/json'],
    CURLOPT_POSTFIELDS => json_encode(['accountId' => 'acc_237018', 'attachments' => [['contentBase64' => 'SGVsbG8gZnJvbSBjYXN0b21BUEk=', 'filename' => 'report.pdf', 'mimeType' => 'application/octet-stream']], 'bcc' => 'example', 'bodyHtml' => '<h1>Hello</h1><p>From castomAPI.</p>', 'bodyText' => 'Hello from castomAPI.', 'cc' => 'example', 'subject' => 'Your September invoice', 'threadId' => 'thr_450968', 'to' => 'grace@example.com']),
]);
$data = json_decode(curl_exec($ch), true);
print_r($data);

Cost and limits

This operation costs 2 tokens. A call that fails is never billed. Every account gets 1000 free tokens a month; paid balances never expire.

Rate limits per minute

No key10
Free60
Pro300
Enterprise1200

Docs → Pricing

Questions

What does Send an email from the connected Google account cost?

2 tokens per successful call, debited from your wallet after the response. Failed calls are free. Every account receives 1000 free tokens a month.

How do I authenticate?

Send your key in the x-api-key header. One key covers every metered endpoint; there is nothing to enable per endpoint.

Can I send an Authorization: Bearer token instead?

Not for a metered endpoint — those take the x-api-key header and nothing else. Bearer tokens belong to the account side: the dashboard and account APIs accept a JWT session, and castomapi-auth issues those through a standard OAuth2 handshake, with authorization code, client credentials and refresh grants. The auth service page covers it.

What are the limits?

10 requests a minute per IP with no key at all, 60 a minute on the free tier, 300 on Pro, 1200 on Enterprise. Above the no-key tier the quota is per account, across every endpoint.

OpenAPI: https://app.castomapi.com/docs/json