How I Streamed a 100 MB Email Attachment in Node.js Without Buffering the Complete File Target Publication : Dev.to / Hashnode / Medium Tags : #nodejs #typescript #webdev #performance #email The Problem with Naive Email Attachment Buffering In standard Node.js email implementations, sending a multi-megabyte email attachment often loads the entire binary file into a Node.js Buffer in memory. When encoding a 100 MB attachment to Base64: The 100 MB raw buffer is loaded into heap memory. Base64 encoding creates a ~133 MB string. MIME line wrapping creates another string object. Total transient memory footprint exceeds 300+ MB for a single attachment! If multiple concurrent requests stream large attachments, Node.js applications frequently experience V8 heap inflation, garbage collection pauses, or Out-Of-Memory (OOM) crashes. The AsyncIterable Streaming Solution To eliminate buffer inflation, MailPort's MIME engine ( @mailport/mime ) processes attachment data incrementally using AsyncIterable streams. 1. Base64 Chunk Boundary Handling Base64 encoding transforms groups of 3 binary bytes into 4 ASCII characters. When streaming arbitrary file chunks (e.g. 64 KB or 10 MB chunks), a chunk boundary may split across a 3-byte tuple. Base64StreamEncoder buffers only the leftover 1 or 2 bytes across chunk boundaries: export class Base64StreamEncoder { # remainder : Uint8Array = new Uint8Array ( 0 ); encodeChunk ( chunk : Uint8Array ): Uint8Array { // Combine 1-2 remainder bytes with incoming chunk // Process in multiples of 3 bytes // Preserve new remainder for next chunk } } 2. Direct Socket Pipeline The generated AsyncIterable stream feeds directly into the TCP/TLS socket write queue with backpressure handling ( socket.write() + drain event listener). Empirical Benchmark Results We benchmarked streaming a 100 MB attachment on Node.js v24 (macOS ARM64): Metric Result Emitted Transmitted Payload 136.84 MB Throughput 387.57 MB/s Duration 353 ms Peak Heap Memory Delta 8.98 MB Peak heap memory usage remained under 9 MB while processing a 136 MB emitted payload! Code Example import { createReadStream } from ' node:fs ' ; import { createMailer , smtp } from ' mailport ' ; const mailer = createMailer ({ transport : smtp ({ host : process . env . SMTP_HOST ! , port : 587 , auth : { user : process . env . SMTP_USER ! , pass : process . env . SMTP_PASSWORD ! }, }), }); await mailer . send ({ from : ' exports@example.com ' , to : ' user@example.com ' , subject : ' Monthly Data Backup ' , attachments : [ { type : ' stream ' , filename : ' backup.sql.gz ' , stream : createReadStream ( ' ./backup.sql.gz ' ), }, ], }); Check out the MailPort repository on GitHub: https://github.com/mahe-gi/mailport