The support forums for BulkWP runs on bbPress and I use wpMandrill to send all emails from BulkWP through the Mandrill App email service from MailChimp. Recently in bbPress v2.5.4 the logic to send topic subscription emails was changed in a major way which broke it for people who were using bbPress with wpMandrill. After spending almost a day debugging I finally got everything to work and thought of documenting it here so that it will be useful for others as well.