Postal is an open-source transactional mail platform originally built by UK hosting company aTech Media (now maintained under the Krystal umbrella) to handle its own outbound and inbound email at scale before being released to the community. It bundles an SMTP server, an HTTP delivery API comparable to Sendgrid or Mailgun, IP pool management for reputation control, and a web dashboard for tracking message logs, bounces, and webhook events. Spam and virus filtering integrates with SpamAssassin and ClamAV/Amavis, and the whole stack deploys via Docker alongside MariaDB and RabbitMQ. Because Postal runs entirely on infrastructure the operator controls, it appeals to companies that want to avoid per-message pricing from commercial email APIs while retaining full ownership of deliverability data and sender reputation.