Message Streams are a feature in Postmark that helps optimize deliverability for promotional emails. To use Message Streams, users need to create a dedicated Broadcast Stream, which is classified as transactional or broadcast based on the type of email being sent. Promotional emails include limited-time offers, sales campaigns, event announcements, discounts, and coupons, among others. Sending these types of messages without an explicit opt-in can lead to lower engagement, spam complaints, and poor deliverability. To set up a Broadcast Stream in Postmark, users can follow quick setup instructions and refer to best practices checklist for optimal results.