Drupal hook: hook_mail_alter

When sending e-mails or newsletters from your website, you might want to inject some information like website name, disclaimer, copy right information, unsubscribe link and such. Just like hook_form_alter, you can use hook_mail_alter to virtually alter anything in the mail just before Drupal sends the e-mail. You can even change the mime type of the email, for example, from plain text to HTML and vice versa.
Here is the syntax/signature of the hook

• Drupal 5: hook_mail_alter(&$mailkey, &$to, &$subject, &$body, &$from, &$headers)
• Drupal 6, 7: hook_mail_alter(&$message)

$message has the following key-value pairs:

• 'id': Unique id of the mail being sent.
• 'to': The destination e-mail address(es)
• 'subject': The subject of the e-mail. This should not contain any new line characters
• 'body': This is an array which contains the actual message to be sent
• 'from': The e-mail address of the sender. This can be either a custom address or site's email address
• 'headers': This is an array containing e-mail's MIME type, Content-type etc.
• 'params': Params are the parameters which is supplied by the function which calls the drupal_mail(), which builds the message prior to hook_mail_alter is invoked.
• 'language': The language used to build the message prior to hook_mail_alter is invoked