If you want to protect your deliverability, it’s essential to separate your transactional and promotional email using Postmark's Message Streams. Transactional streams handle invoices, password reset emails, while broadcast message streams are for newsletters and promotions. To process mail fast and reliably, you must specify whether the email is a transactional or broadcast message. When sending via SMTP, users could previously only use a single option to specify a Message Stream by authenticating with a Server Token and setting the `X-PM-Message-Stream` header. However, this was limited by some SMTP clients not allowing custom headers. To fix this issue, Postmark introduced Custom SMTP Tokens for all Message Streams, which allow users to authenticate and send email via their chosen Message Stream without modifying email headers. These tokens can be generated for each outgoing Message Stream, providing an additional layer of security and flexibility.