Does it work with Mailpoet/Wisija?

You need to enable MailPoet’s compatibility mode.

Stick this into your theme’s functions.php file:

add_action( 'init', 'mailpoet_hidden_options');
function mailpoet_hidden_options(){
    if(class_exists('WYSIJA')){
        $model_config = WYSIJA::get('config','model');
        $model_config->save( array( 'allow_wpmail' => true ));
    }
}

Then select WP Mail as the sending method in Mail Poet.

thanks a lot for the quick reply!your method partially worked,
now it send the messages but the content of the message is blank.
(internal server error 500).

Internal server error means your PHP crashed. That’s a problem I can’t help you with, though enabling WordPress Debug might show you where the problem occurred.

Thanks a lot.
I’m Writing to the guys at Mail Poet and to my hosting service.
Hope to solve this.
Best.

Just a quick update.
The guys at MAILPOET told me that the plug in it’s not able to work with
wp mail, so if you try to use this option with the function hack up here, the effect it’s a blank email.
The only working solutions is to change Newsletter plug in or to use the Sendgrid integration on MAILPOET.
That’s all.

Well, that’s the opposite of what they told me!

Glad you found a solution.