The Postmark SMTP Field Manual is an open-source app designed to provide transparency about SMTP bounce responses from various mail servers. It offers a comprehensive list of error codes, severity flags, and actionable advice for troubleshooting deliverability issues. The manual has been improved through community feedback since its launch in 2019 and allows users to contribute their expertise by submitting pull requests or sharing their knowledge via email. By utilizing this resource, users can gain a better understanding of SMTP responses and take targeted steps to resolve issues that affect email delivery.