MailChecker is a cross-language email validation library that detects emails from temporary/disposable email providers, distributed as first-class packages for Node.js, PHP, Python, Ruby, Rust, Go, Elixir, Clojure, browser JavaScript, and Java. All platforms share one curated plain-text blacklist (list.txt) of ~55,000 disposable domains and expose a normalized API — isValid/valid?/is_valid, blacklist(), and addCustomDomains. Each implementation combines a strict RFC-style regex format check with suffix-based blacklist matching, so subdomains of known disposable domains (e.g. [email protected]) are also rejected. The npm package is deliberately dependency-free, using only a Set and a single anchored regex at runtime. The defining architectural pattern is template-driven code generation: lib/clean.js deduplicates and validates list.txt against the public suffix list, lib/loader.js reads it, and lib/generator.js renders *.tmpl.* files into final platform sources, meaning a single PR that edits list.txt propagates to every language distribution after a build. Tests exist for all major platforms and CI runs them in Docker for Node, PHP, Python, Ruby, Rust, Go, and Elixir. Maintenance is genuinely active, with v6.0.21 released in July 2026 and continuous community PRs adding/removing domains. However, the toolchain lags: test images pin EOL runtimes (Python 2.7/3.6, Ruby 2.2, Go 1.13), the abandoned mu2 template engine powers the generator, the Clojure test is disabled because the emitted source is too large for the compiler, and the Java platform loads list.txt from classpath resources but no such resource file is committed, making that platform effectively non-functional as distributed. The core Node, PHP, Python, Ruby, Go, and Rust implementations are solid, idiomatic, and well-tested.