# boolean_smtp_mail_headers

> Filters the header bag built from the message before normalization.

Filter · Tier A · Since 1.0.0

Return a MailHeaders instance, or an array of header lines or name/value pairs, to replace it.

## Signature

```php
add_filter( 'boolean_smtp_mail_headers', $callback, 10, 2 );
```

## Parameters

| Name | Type | Description |
| --- | --- | --- |
| `$headers` | `\BooleanSmtp\Services\Mailer\Headers\MailHeaders` | Header bag parsed from the message's raw headers. |
| `$message` | `\BooleanSmtp\Services\Mailer\MailMessage` | Message the headers belong to. |

**Returns** `\BooleanSmtp\Services\Mailer\Headers\MailHeaders` — The filtered headers.

## Fires in

- [app/Services/Mailer/BooleanSmtpMailPipeline.php:311](https://github.com/booleanpress/boolean-smtp-src/blob/main/app/Services/Mailer/BooleanSmtpMailPipeline.php#L311) — `BooleanSmtp\Services\Mailer\BooleanSmtpMailPipeline::applyHeaderPipeline()`

## Example

```php
add_filter( 'boolean_smtp_mail_headers', function ( \BooleanSmtp\Services\Mailer\Headers\MailHeaders $headers, \BooleanSmtp\Services\Mailer\MailMessage $message ) {
    return $headers;
}, 10, 2 );
```