Emails are filtered for HTML codes. The <span> tag is not allowed in email content as it is not a Post content. It should be secure and clean. This will help emails to pass more antispam systems of Email servers.

The default WordPress emails doesn't have HTML tags, we added a few secure tags (formatting tags and link).

By default, allowed tags are: 'a', 'img', 'blockquote', 'h1-h5','hr', 'br',  'p', 'style', 'strong'.

However, you can use the below code to allow all tags to work.

Put this code in your active theme functions.php file:

function my_custom_email_html_tags( $allowed_html ){
    if( $allowed_post_html = wp_kses_allowed_html( 'post' ) ){
        $allowed_html = $allowed_post_html;
    }
    return $allowed_html;
}
add_filter( 'wpforo_kses_allowed_html_email', 'my_custom_email_html_tags');

How to Easily Add Custom Code in WordPress (without Breaking Your Site)

Examble Set Font Size 16 bevor saving

<p style="font-family: arial;"><span style="font-size: 16px;">Hallo ,</span></p><p><span style="font-size: 16px;">in Ihrem abonnierten Fachthema wurde ein neues Thema erstellt.</span></p>

after saving

<p style="font-family: arial;">Hallo ,</p><p>in Ihrem abonnierten Fachthema wurde ein neues Thema erstellt.</p>