This is a simplified version of accessing some values (sender, receiver, cc, bcc, attachments size...) of mails inside mbox file.

simplified-version-of-reading-mbox-file.js

import nodeMbox from 'node-mbox';
import { simpleParser } from 'mailparser';
import { createReadStream } from 'node:fs';

const mboxFilePath = './All mail Including Spam and Trash.mbox';

const scanHeaders = (headersMap) => {
  const headers = headersMap;

  const init_From = headers.get('from');
  const init_To = headers.get('to');

  const init_cc = headers.get('cc');
  const init_bcc = headers.get('bcc');
  //
  const init_date = headers.get('date');
  const init_messageId = headers.get('message-id');

  const initialMainInfoForThisMail = {
    from: init_From,
    to: init_To,

    cc: init_cc,
    bcc: init_bcc,
    //
    date: init_date,
    ['message-id']: init_messageId,
  };

  return initialMainInfoForThisMail;
};

const processParsedMail = (theParsedMail) => {
  const initialMainInfoForThisMail = scanHeaders(theParsedMail.headers);

  // bytes
  const sumOfSizesOfAttachmentsOfOneMail = theParsedMail.attachments.reduce(
    (accu, curr) => accu + curr.size,
    0,
  );

  const countOfAttachmentsInThisMail = theParsedMail.attachments.length;

  // do something with:
  //      initialMainInfoForThisMail
  //      sumOfSizesOfAttachmentsOfOneMail
  //      countOfAttachmentsInThisMail
};

const processOneMail = async (rawMailBuffer) => {
  const parsedMail = await simpleParser(rawMailBuffer, {});

  processParsedMail(parsedMail);
};

const analyzeMbox = () => {
  const mailbox = createReadStream(mboxFilePath);
  const mbox = nodeMbox.MboxStream(mailbox, {
    /* options */
  });

  mbox.on('data', function (msg) {
    try {
      processOneMail(msg);
    } catch (err) {
      console.log(err);
    }
  });

  mbox.on('error', function (err) {
    console.log('got an error', err);
  });

  mbox.on('finish', async function () {
    console.log(
      `Finished accessing all the mails, but not yet finished proccessing them. Please wait.`,
    );

    // Wait for all proccessings to finish, and then do something
  });
};

analyzeMbox();