Mail not sent when I set HTML headers

There’s the wp_mail() function in WordPress. The headers have to be added as array without trailing \n\r or similar.

Example

wp_mail(
    '[email protected]',
    'Hello World!',
    'Just saying...',
    array(
        'MIME-Version: 1.0',
        'Content-type: text/html; charset=iso-8859-1',
        sprintf(
            'From: %s <no-reply@%s>',
            get_bloginfo('name'),
            site_url()
        ),
        sprintf( 'X-Mailer: PHP/%s', phpversion() ),
     )
);

To change the content type you could as well use a filter:

<?php
/* Plugin Name: WP Mail Content Type text/html */
function wpse_97789_mail_contenttype( $content_type )
{
    remove_filter( current_filter(), __FUNCTION__ );
    return 'text/html';
}

// Then, whereever you need it, just add the filter before calling the function
// It removes itself after firing once
add_filter( 'wp_mail_content_type', 'wpse_97789_mail_contenttype' );
wp_mail(
    '[email protected]',
    'Hello World!',
    'Just saying...',
    array(
        'MIME-Version: 1.0',
        sprintf(
            'From: %s <no-reply@%s>',
            get_bloginfo('name'),
            site_url()
        ),
        sprintf( 'X-Mailer: PHP/%s', phpversion() ),
    )
);