Mails are send like text/plain (conflict with "SMTP Authentication Support" module or miss something in documentation ?)

Hi,

I've read the documentation, and maybe I don't understand everything, but I've a little problem : all my mails are send as "text/plain".

I've copied htmlmail.tpl.php to my theme directory.
In admin/config/system/htmlmail I've choose "my theme" in "step 2".
And in "step 3" I use a "Post-filtering" that I've created for mails (all HTML authorised).

I have no choice : I need to use https://www.drupal.org/project/smtp to send mail.
I've checked " Allow to send e-mails formatted as Html " in /admin/config/system/smtp.

In admin/config/system/mailsystem I've choose :
Site-wide default "MailSystemInterface" class : SmtpMailSystem
HTML Mail module class : SmtpMailSystem (I've tryed with HTMLMailSystem too, but same problem).
Theme to render the emails : Active theme (my theme).

When I send a test mail from /admin/config/system/htmlmail/test, I receive something like that :

To: to@me.com
From: from@me.com
Subject: essai
Message-ID: <36c280c0502aca7f9f95107@myip>
X-Priority: 3
X-Mailer: PHPMailer 5.1 (phpmailer.sourceforge.net)
Sender: from@me.com
MIME-Version: 1.0
Content-Transfer-Encoding: 8Bit
Content-Type: text/plain; charset="utf-8"

<p>testing <strong>Mail by HTML</strong></p>
<p></p>

All my HTML code is displayed in text format. I think "Content-Type" should be set to "text/html", not "text/plain"... But how to do it ?

What I've doing wrong ?
Please, help !