…dcms#15645)

After some trying (and failing) to enable unsubscribe headers, I found
that the Resend adapter does not pass custom headers from the Payload
sendEmail options to the Resend API. When attempting to send an email
with custom headers, they are ignored because the
`mapPayloadEmailToResendEmail` function creates a new object that
excludes the headers property.

```
await payload.sendEmail({
  from: "Test <test@domain.com>",
  to: "jimmybillbob@example.com",
  subject: "Resend adapter does not allow custom headers :(",
  html: html,
  headers: {
    "List-Unsubscribe": "<https://domain.com/unsubscribe>",
    "List-Unsubscribe-Post": "List-Unsubscribe=One-Click",
  },
});
```

I have recitified this by adding `headers: message.headers,` to the
`mapPayloadEmailToResendEmail` in `packages/email-resend/src/index.ts`,
as below.

```
function mapPayloadEmailToResendEmail(
  message: SendEmailOptions,
  defaultFromAddress: string,
  defaultFromName: string,
): ResendSendEmailOptions {
  return {
    // Required
    from: mapFromAddress(message.from, defaultFromName, defaultFromAddress),
    subject: message.subject ?? '',
    to: mapAddresses(message.to),

    // Other To fields
    bcc: mapAddresses(message.bcc),
    cc: mapAddresses(message.cc),
    reply_to: mapAddresses(message.replyTo),

    // Optional
    attachments: mapAttachments(message.attachments),
    html: message.html?.toString() || '',
    text: message.text?.toString() || '',
    headers: message.headers, // Added this line.
  } as ResendSendEmailOptions
}
```

---------

Co-authored-by: Paul Popus <paul@payloadcms.com>

…dcms#15645)

After some trying (and failing) to enable unsubscribe headers, I found
that the Resend adapter does not pass custom headers from the Payload
sendEmail options to the Resend API. When attempting to send an email
with custom headers, they are ignored because the
`mapPayloadEmailToResendEmail` function creates a new object that
excludes the headers property.

```
await payload.sendEmail({
  from: "Test <test@domain.com>",
  to: "jimmybillbob@example.com",
  subject: "Resend adapter does not allow custom headers :(",
  html: html,
  headers: {
    "List-Unsubscribe": "<https://domain.com/unsubscribe>",
    "List-Unsubscribe-Post": "List-Unsubscribe=One-Click",
  },
});
```

I have recitified this by adding `headers: message.headers,` to the
`mapPayloadEmailToResendEmail` in `packages/email-resend/src/index.ts`,
as below.

```
function mapPayloadEmailToResendEmail(
  message: SendEmailOptions,
  defaultFromAddress: string,
  defaultFromName: string,
): ResendSendEmailOptions {
  return {
    // Required
    from: mapFromAddress(message.from, defaultFromName, defaultFromAddress),
    subject: message.subject ?? '',
    to: mapAddresses(message.to),

    // Other To fields
    bcc: mapAddresses(message.bcc),
    cc: mapAddresses(message.cc),
    reply_to: mapAddresses(message.replyTo),

    // Optional
    attachments: mapAttachments(message.attachments),
    html: message.html?.toString() || '',
    text: message.text?.toString() || '',
    headers: message.headers, // Added this line.
  } as ResendSendEmailOptions
}
```

---------

Co-authored-by: Paul Popus <paul@payloadcms.com>