The 6,000-Line Email Parser

Andris Reinman, a developer in Tallinn, Estonia, single-handedly built the entire Node.js email ecosystem over a decade, including Nodemailer, Mailparser, WildDuck, and ZoneMTA. Mailparser, which receives 75 million downloads annually, handles the extraordinarily complex task of parsing emails — navigating 40 years of overlapping RFCs, nested MIME structures, multiple character encodings, and countless non-standard vendor implementations. The parser is architected as a pipeline of Node.js Transform streams, using a byte-level state machine to split emails into MIME node trees and applying sequential decoding steps for each part.