Postmark-PHP is an official PHP Client for the Postmark API, offering complete coverage of the API, easy installation via Composer, comprehensive documentation with extensive examples, support for multiple PHP runtimes, and test coverage. It is open source, allowing users to browse the source code and contribute enhancements. The library aims to make using Postmark easier by providing a strong foundation for creating adapters for popular frameworks such as Laravel, Symfony2Swiftmailer, and ZendMail, which can be contributed to by the community.