Attachments

attachments is an array of up to 50 objects. Each attachment needs a filename and exactly one of content or path.

filenamestringrequired

1 to 255 characters. Shown to the recipient.

contentstringoptional

File bytes as base64. The SDK also accepts a Buffer/Uint8Array and encodes it for you. Use this or path.

pathstringoptional

A public https URL. SpacePost downloads it when the email is sent. Use this or content.

content_typestringoptional

MIME type, such as application/pdf. Alias contentType in the SDK.

content_idstringoptional

Makes the attachment inline, referenceable as cid:<content_id> in your HTML. Alias contentId in the SDK.

Limits

• The whole message, including all attachments after base64 encoding, must be under 40 MB. Base64 inflates files by about 33%, so keep the raw total under roughly 30 MB.
• URL attachments (path) must use https, must resolve to a public address (private networks are refused), are fetched with a 10 second timeout and must not redirect. Fetch failures mark the email failed.
• Attachment bytes are removed from storage once the email is sent; only metadata (name, type, size) is kept in the log.
• Attachments don’t cost extra on any standard plan. SpacePost is built for transactional and marketing email, not bulk file delivery: if your attachment volume is unusually high compared with your email volume, we may contact you about a custom plan (see the Acceptable Use Policy).

Attach a file by content

Node.js

attachment.ts

import { readFile } from "node:fs/promises";
import { SpacePost } from "@spacepost/node";

const spacepost = new SpacePost(process.env.SPACEPOST_API_KEY);

const { data, error } = await spacepost.emails.send({
  from: "Acme <billing@acme.com>",
  to: "user@example.com",
  subject: "Invoice 1042",
  text: "Your invoice is attached.",
  attachments: [{ filename: "invoice-1042.pdf", content: await readFile("invoice-1042.pdf"), contentType: "application/pdf" }],
});

console.log(error ?? data);

cURL

curl -X POST https://api.spacepost.dev/emails \
  -H "Authorization: Bearer $SPACEPOST_API_KEY" -H "Content-Type: application/json" \
  -d "{
    \"from\": \"Acme <billing@acme.com>\",
    \"to\": [\"user@example.com\"],
    \"subject\": \"Invoice 1042\",
    \"text\": \"Your invoice is attached.\",
    \"attachments\": [{
      \"filename\": \"invoice-1042.pdf\",
      \"content_type\": \"application/pdf\",
      \"content\": \"$(base64 -w0 invoice-1042.pdf)\"
    }]
  }"

On macOS use base64 -i invoice-1042.pdf (no -w0).

Python

import base64

with open("invoice-1042.pdf", "rb") as f:
    content = base64.b64encode(f.read()).decode()

requests.post(
    "https://api.spacepost.dev/emails",
    headers={"Authorization": f"Bearer {os.environ['SPACEPOST_API_KEY']}"},
    json={
        "from": "Acme <billing@acme.com>",
        "to": ["user@example.com"],
        "subject": "Invoice 1042",
        "text": "Your invoice is attached.",
        "attachments": [{"filename": "invoice-1042.pdf", "content_type": "application/pdf", "content": content}],
    },
)

Attach a file by URL

{
  "from": "Acme <billing@acme.com>",
  "to": ["user@example.com"],
  "subject": "Your report",
  "text": "Report attached.",
  "attachments": [{ "filename": "report.pdf", "path": "https://files.acme.com/reports/2026-10.pdf" }]
}

Caution

Giving both content and path, or neither, returns 422: “Each attachment needs exactly one of content or path”.

Inline images

Set a content_id and reference it with a cid: URL in your HTML.

inline-image.ts

import { readFile } from "node:fs/promises";
import { SpacePost } from "@spacepost/node";

const spacepost = new SpacePost(process.env.SPACEPOST_API_KEY);

await spacepost.emails.send({
  from: "Acme <hello@acme.com>",
  to: "user@example.com",
  subject: "Welcome",
  html: '<img src="cid:logo" alt="Acme" width="120" /><p>Welcome aboard.</p>',
  attachments: [{ filename: "logo.png", content: await readFile("logo.png"), contentType: "image/png", contentId: "logo" }],
});

Inline images are sent as part of the message and display without a network request, but many clients (Gmail on the web in particular) show them best when they are small. For large images host them on your own CDN and use a normal https:// URL.