Abstract base class for the stream filter pipeline pattern. Implements the IMimeFilter interface with Filter() and Flush() methods, allowing composable transforms (encoding, line-ending conversion, whitespace stripping) to be chained in FilteredStream. The pattern is cleanly separated and could be extracted for any stream transformation use case.

A full HTML5 tokenizer implemented as a state machine, mirroring the HTML5 specification states. It tokenizes HTML into tokens (tags, attributes, comments, CDATA, script data) with proper handling of self-closing tags, foreign elements, and character references. This is a non-trivial, self-contained component that could be reused in any HTML processing pipeline.

A ref struct that provides efficient string building using ArrayPool<char> to minimize allocations. Supports appending, formatting, and inserting with minimal overhead. This is a reusable utility pattern for any performance-sensitive .NET code that needs to build strings without GC pressure.

A stream wrapper that restricts reads and writes to a subrange of an underlying stream. Used by MimeParser to lazily access MIME part content without loading the entire message into memory. The implementation is self-contained and could be extracted for any scenario requiring bounded I/O on a stream.

Base class for DKIM signature verification. Implements the core verification algorithm including header canonicalization, body hash computation, and signature validation. The class is designed to be extended with custom public key locators and provides both sync and async verification methods. Demonstrates a clean separation of concerns for cryptographic verification.