Postmark's Message Streams offer a way to separate promotional and transactional email, which can improve deliverability. Separating these types of emails helps avoid issues with spam complaints or deliverability problems caused by sending promotional emails through the same IP pool as transactional emails. Postmark also provides features such as Message Stream servers for each client/customer, allowing for better organization and tracking of sending statistics. Additionally, users can use sub-domains to separate their transactional and bulk Broadcast sending, which helps develop a separate sending reputation for each domain. Postmark has a 99% deliverability rate and offers resources on best practices for sending emails, as well as information on GDPR compliance when sending from the EU.