Message Streams is a new concept introduced by Postmark to help customers organize and separate their sending within a server, reducing the proliferation of servers on their dashboard. The new feature allows users to create two default Message Streams: transactional and inbound streams, which can be managed through the Postmark UI. This update aims to improve deliverability and reputation for transactional email while separating bulk message sending from other types of emails. The feature is part of a larger effort by Postmark to expand its capabilities beyond transactional email and make it easier for customers to send all their customer communications through the platform. With this change, users can expect more control over their email sending and improved deliverability, as well as access to additional API calls and endpoints for managing custom Message Streams in the future.