Thanks Robin for a great help. I am coding my own theme, and code from this plugin helped me to find out the filter ‘bbp_subscription_mail_message’. Though function ‘bbp_pre_notify_subscribers’ and ‘bbp_post_notify_subscribers’ are not working. But its working great with the ‘wp_mail_content_type’. It is now returning all mails from wordpress to Html emails. Anyway atleast it solved my issue. Thanks!!