bbPress.org

It may well be the address that bbpress is sending from.

this is typically noreply@ and if this email address does not exist then many email hosts will not permit its transit through the system.

either

bbp style pack

once activated go to

dashboard>settings>bbp style pack>Subscription Emails

where you can amend the address and change the wording as well

or

Put this in your child theme’s function file –

ie wp-content/themes/%your-theme-name%/functions.php

where %your-theme-name% is the name of your theme

or use

Code Snippets

add_filter('bbp_get_do_not_reply_address','my_bbp_no_reply_email');
function no_reply_email(){
    $email = 'noreply@yourdomain.com'; // any email you want
    return $email;
}

and change the address