hook_mail_alter() doesn't remove the Return-Path, Sender, and Reply-To headers

I want to remove the Return-Path, Sender, and Reply-To headers from the sent emails. I am trying to achieve this using the following code, but the headers aren't removed from the email.

function mymodule_mail_alter(&$message) {
  unset($message['headers']['Return-Path']);
  unset($message['headers']['Sender']);
  unset($message['headers']['Reply-To']);
}

What am I doing wrong?

Removing the values from $message['headers'] isn't sufficient, as the plugin used to send emails could also use $message['Return-Path'], $message['Sender'], or $message['Reply-To'] value to build the email headers. The class that is used as default plugin to send emails, PhpMail uses the following code, but other plugins could use different code.

$site_mail = $this->configFactory->get('system.site')->get('mail');
$additional_headers = isset($message['Return-Path']) && ($site_mail === $message['Return-Path'] || static::_isShellSafe($message['Return-Path'])) ? '-f' . $message['Return-Path'] : '';
$mail_result = @mail($message['to'], $mail_subject, $mail_body, $mail_headers, $additional_headers);

Since the email plugin is used to send the email after the hook_mail_alter() implementations are invoked, and since the email plugin can add one of those headers to its discretion, there isn't any warranty the email is sent without those headers. Assuming the email plugin adds only those headers when $message or $message['headers'] contains a value for them, the following code should work.

function mymodule_mail_alter(&$message) {
  unset($message['Return-Path']);
  unset($message['Sender']);
  unset($message['Reply-To']);
  unset($message['headers']['Return-Path']);
  unset($message['headers']['Sender']);
  unset($message['headers']['Reply-To']);
}

The SMTP module always sets certain headers which cannot be unset. See the following lines from Drupal\smtp\Plugin\Mail\SMTPMailSystem:

$headers['Sender'] = $from;
$headers['Return-Path'] = $from;
$headers['Reply-To'] = $from;

This is to make it compatible with Drupal core which does the same from what I can tell.

For more control, you can use the module PHPMailer SMTP instead which explicitly unsets the Return-Path header to comply with RFC 5321.

It also makes no assumptions about which extra headers should be present (beyond the required ones) and only sets them if they are added elsewhere.

The following would work if the case of the headers match:

function mymodule_mail_alter(&$message) {
  unset($message['headers']['Sender']);
  unset($message['headers']['Reply-To']);
}

Alternatively, the following would unset headers regardless of case:

function mymodule_mail_alter(&$message) {
  $message['headers'] = array_change_key_case($message['headers']);
  unset($message['headers']['sender']);
  unset($message['headers']['reply-to']);
}

PHPMailer SMTP is for sending email only and does not do any HTML formatting so you'd also need to use a module such as MIME Mail for that purpose.

Disclaimer: I am the author of PHPMailer SMTP.

Use function hook_mail($key, &$message, $params) instead. Also, you would need additional $key I believe to distinguish for which functionality is the mail being sent.

Then this hook would be triggered when sending the mail manually:

$send = true;
$mailManager = \Drupal::service('plugin.manager.mail');
$result = $mailManager->mail($module, $key, $to, $langcode, $params, null, $send);

I would suggest using Swiftmailer, it omits all of the manual work.