# SDK field support (/docs/adapters/field-support)



Email SDK keeps one message shape, but email APIs do not all expose the same features. This page documents what each Email SDK adapter maps today. Stable adapters either map a field or reject it with a validation error. They should not silently drop message data.

## How to read this page [#how-to-read-this-page]

* `Yes` means the adapter maps that `EmailMessage` field.
* `No` means the adapter rejects that field before calling the provider.
* `Values` means Email SDK sends each tag's value to the provider.
* `First tag` means only the first tag can be represented by that provider API.

## Best fits [#best-fits]

| Need                             | Start with                                         |
| -------------------------------- | -------------------------------------------------- |
| Most complete API field mapping  | Postmark, SendGrid, Mailgun, Brevo                 |
| Resend-style DX with attachments | Resend                                             |
| Cheap or self-managed delivery   | SMTP                                               |
| Product/event email              | Loops, Plunk                                       |
| Provider fallback for production | Resend plus SMTP, or one primary API plus Postmark |
| Attachment-heavy transactional   | Resend, Postmark, SendGrid, Mailgun, MailerSend    |

## API adapters [#api-adapters]

These adapters cover most transactional email fields. They are the best fit when your app needs CC, BCC, reply-to, custom headers, tags, metadata, or attachments.

| Adapter                 | CC  | BCC | Reply-To | Headers | Metadata | Tags      | Attachments |
| ----------------------- | --- | --- | -------- | ------- | -------- | --------- | ----------- |
| Resend                  | Yes | Yes | Yes      | Yes     | No       | Yes       | Yes         |
| Postmark                | Yes | Yes | Yes      | Yes     | Yes      | First tag | Yes         |
| SendGrid                | Yes | Yes | Yes      | Yes     | Yes      | Values    | Yes         |
| Mailgun                 | Yes | Yes | Yes      | Yes     | Yes      | Values    | Yes         |
| MailerSend              | Yes | Yes | Yes      | Yes     | No       | Values    | Yes         |
| Brevo                   | Yes | Yes | Yes      | Yes     | Yes      | Values    | Yes         |
| Mailchimp Transactional | Yes | Yes | No       | Yes     | Yes      | Values    | Yes         |
| Mailtrap                | Yes | Yes | No       | Yes     | No       | First tag | Yes         |

## Narrow adapters [#narrow-adapters]

These adapters are useful, but their public APIs do not match the full `EmailMessage` shape. Email SDK keeps them stable by rejecting fields it cannot represent.

| Adapter   | CC  | BCC | Reply-To | Headers | Metadata | Tags   | Attachments |
| --------- | --- | --- | -------- | ------- | -------- | ------ | ----------- |
| SparkPost | No  | No  | Yes      | Yes     | Yes      | Values | Yes         |
| Loops     | No  | No  | No       | No      | Yes      | No     | No          |
| Plunk     | No  | No  | No       | No      | Yes      | No     | No          |
| Scaleway  | Yes | Yes | No       | Yes     | No       | No     | No          |
| ZeptoMail | Yes | Yes | Yes      | No      | No       | No     | Yes         |
| MailPace  | Yes | Yes | Yes      | No      | No       | No     | No          |

## SMTP transport [#smtp-transport]

SMTP is built in and does not use Nodemailer. It maps address fields and headers directly to the SMTP message, but it does not map provider-only concepts like tags or metadata.

| Adapter | CC  | BCC | Reply-To | Headers | Metadata | Tags | Attachments |
| ------- | --- | --- | -------- | ------- | -------- | ---- | ----------- |
| SMTP    | Yes | Yes | Yes      | Yes     | No       | No   | No          |

## Validation behavior [#validation-behavior]

Unsupported fields fail fast. For example, Resend rejects `metadata`, Mailchimp Transactional rejects `replyTo`, and SMTP rejects attachments. That keeps production behavior boring: if a field matters, you find out before the provider request.

## Attachment rules [#attachment-rules]

Attachment `content` is treated as raw content by default and encoded to Base64 for API adapters that require Base64.

```ts
attachments: [
  {
    filename: "receipt.txt",
    content: "Thanks for your order.",
    contentType: "text/plain",
  },
];
```

If you already have Base64 content, mark it:

```ts
attachments: [
  {
    filename: "receipt.pdf",
    content: base64Pdf,
    contentEncoding: "base64",
    contentType: "application/pdf",
  },
];
```

Adapters that support attachments can also read a local path:

```ts
attachments: [
  {
    filename: "receipt.pdf",
    path: "./receipt.pdf",
    contentType: "application/pdf",
  },
];
```