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The maxSize plugin enforces message size limits by rejecting emails that exceed a specified byte threshold. This helps protect your server from resource exhaustion and ensures compliance with message size policies.

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What It Does

The maxSize plugin:
  • Monitors incoming message size during the DATA phase
  • Rejects messages that exceed the configured byte limit
  • Returns an RFC-compliant 552 error code when the limit is exceeded

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Function Signature

function maxSize(bytes: number): Plugin

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Parameters

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bytes
number
required
Maximum message size in bytes. Messages exceeding this limit will be rejected with a 552 error code.

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Usage

Import and configure the maxSize plugin with your desired byte limit: 
import { Fumi } from "@puiusabin/fumi";
import { maxSize } from "@puiusabin/fumi/plugins/max-size";

// Create Fumi instance with size tracking enabled
const app = new Fumi({ 
  size: 1_000_000  // 1 MB
});

// Add the maxSize plugin with the same limit
app.use(maxSize(1_000_000));

await app.listen(2525);
Important: You must set FumiOptions.size to the same byte value when creating your Fumi instance. This enables the underlying SMTP library to track message size and set the sizeExceeded flag on the stream.
const app = new Fumi({ size: 1_000_000 });
app.use(maxSize(1_000_000));
If FumiOptions.size is not set, the plugin will not function correctly.

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Example with Different Limits

import { Fumi } from "@puiusabin/fumi";
import { maxSize } from "@puiusabin/fumi/plugins/max-size";

// 5 MB limit
const FIVE_MB = 5 * 1024 * 1024;

const app = new Fumi({ size: FIVE_MB });
app.use(maxSize(FIVE_MB));

await app.listen(2525);

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Error Response

When a message exceeds the size limit, clients receive: 
552 Message exceeds the maximum size of 1000000 bytes
The 552 status code is the standard SMTP code for “message exceeds fixed maximum message size”.

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Implementation

The plugin hooks into the onData event and checks the sizeExceeded flag after the message stream is consumed: 
export function maxSize(bytes: number): Plugin {
  return (app) => {
    app.onData(async (ctx, next) => {
      await next();
      await ctx.stream.pipeTo(new WritableStream());
      if (ctx.sizeExceeded) {
        ctx.reject(`Message exceeds the maximum size of ${bytes} bytes`, 552);
      }
    });
  };
}

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Common Size Limits

  • 1 MB: 1_000_000 or 1 * 1024 * 1024
  • 5 MB: 5_000_000 or 5 * 1024 * 1024
  • 10 MB: 10_000_000 or 10 * 1024 * 1024
  • 25 MB: 25_000_000 or 25 * 1024 * 1024 (Gmail’s limit)

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