Postmark is a transactional email service that also allows customers to send bulk and marketing emails. The company's unique infrastructure routes different types of email through separate Message Streams, which helps maintain optimal deliverability. To send promotional emails with Postmark, customers need to create a dedicated Broadcast Stream and ensure they are sending messages to recipients who have explicitly opted-in to receive them. This approach can help improve engagement, reduce spam complaints, and enhance the overall reputation of the sender.