Compatibility with Reroute Email

First of all, we need to understand how Drupal sends e-mails:

Drupal can handle multiple systems to send mails. There is a system variable (variable_get('mail_system') with the enabled systems to send e-mails. By default, there is only one, called DefaultMailSystem ( https://api.drupal.org/api/drupal/modules%21system%21system.mail.inc/cla... ). Those "systems" are classes that implement a default interface called MailSystemInterface ( https://api.drupal.org/api/drupal/includes%21mail.inc/interface/MailSyst... ).

This interface only requires two functions: format and mail. The first one handles the formating and the second one, sends it.

So, what causes your problem?

If you implement a mail system, you have to implement both methods, the format and the mail function. The HTML Mail module actually "only needs" to do the formating, meanwhile Reroute Email "only needs" the sending part. HTML Mail doesn't know that you have another system to send e-mails and Reroute Email doesn't know that you have another system to format the e-mails.

Is there a solution for this?

Yes, there is. You can implement your own MailSystemInterface in a custom class that does the formating of HTML Mail and the sending of Reroute Email. If you don't want to do this, the Mail System module has a functionality that does it for you.

Just go to admin/config/system/mailsystem and under "New Class" select for the format method the HTML Mail and for the mail method the Reroute Email and save the settings. This will generate a new class, called HTMLMailSystem__RerouteEmailSystem or something like that.

Ok, nevermind, just reviewed how Reroute Email works and doesn't implement it's own class. Sorry, I should have read better the issue description.

In the hook_mail_alter of the reroute_email module, there is only one place where the $message['to'] is altered, to reroute the e-mail:

    if (!in_array($to, $addresslist)) {
      // Not on the list, so reroute to the first address in the list.
      $message['headers']['X-Rerouted-Original-To'] = $to;
      $message['to'] = !empty($addresslist[0]) ? $addresslist[0] : REROUTE_EMAIL_ADDRESS_EMPTY_PLACEHOLDER;

So, unless the if condition is not being applied, I don't see why the reroute_email will not work with HTMLMail.

I'm trying this, to see what's going on.

I should have started here:

Installed HTML Mail and Reroute Email. Enabled both modules, tried the send test email from html mail at admin/config/system/htmlmail/test

MIME-Version: 1.0
Content-Type: text/html; charset=utf-8
Content-Transfer-Encoding: 8Bit
X-Mailer: Drupal
Return-Path: info@example.com
Sender: info@example.com
From: "Info" <info@example.com>
X-Rerouted-Mail-Key: htmlmail_test
X-Rerouted-Website: http://drupal.local
X-Rerouted-Original-To: original@example.com
To: rerouted@example.com
Subject: test

<h1><a href="http://drupal.org/project/htmlmail">HTML Mail</a> test message</h1>
<div class="htmlmail-body">
This email was rerouted.
Web site: http://drupal.local
Mail key: htmlmail_test
Originally to: original@example.com
-----------------------
<br />
<br />
<p>test</p>
</div>

Can someone else verify that it works correctly?

No patch and not sufficient interest, sorry...