When signing up for Postmark, users are required to use a work email address from a private domain, as public domains like Gmail or Yahoo are not allowed due to concerns about spoofing and preventing bad email behavior. This policy helps maintain deliverability and prevents unauthorized use of domains. The domain used when signing up provides information about the business and its professionalism, which is verified by Postmark's team before allowing access to the platform. Using public domains can lead to DMARC failures, bounces, and rejections from receivers, making it necessary for Postmark to verify authorized senders.