Behind-the-scenes HTTP Request?

Only use wp_remote_post() if you are actually posting something.

Try using wp_remote_get() with a full url

$url="http://blabla.org/mailinglist/add?" . $user_email;
$results = wp_remote_get( $url );
// var_dump( $results );

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)