getNewMail.js

const fs = require('fs');
const path = require('path');
const s3etm = require('s3-emails-to-mongo');

// MAKE THIS PART DYNAMIC LATER
s3etm.configure({
  Bucket: 'zhillb-mail',
});

module.exports = async (req, res, next) => {
  try {
    const newMail = await s3etm();
    // check if new mail has attachment(s), if so save them
    newMail.forEach((msg) => {
      try {
        if (msg.attachments.length) {
          msg.attachments.forEach(async (attachment) => {
            const file = attachment.content;
            const cid = attachment.cid;
            const filePath = path.join(__dirname, '..', 'data/uploads', cid);
            fs.writeFile(filePath, file, (err, data) => {
              if (err) {
                console.log('err with file: ', err);
              } else {
                console.log('saved file: ' + filePath);
                console.log('mime type: ', attachment.contentType);
              }
            });
          });
        }
      } catch(err) {
        next(err);
      }
    });
    res.json(newMail);
  } catch(err) {
    next(err);
  }
};

this should work equal as well, and should be pretty zippy ;)

const fs = require('fs')
const util = require('util')
const path = require('path')
const s3etm = require('s3-emails-to-mongo')

const directory = path.join(__dirname, '..', 'data', 'uploads')
const writeFile = util.promisify(fs.writeFile)

s3etm.configure({
  Bucket: 'zhillb-mail',
})

module.exports = async (request, response, next) => {
	const mail = await s3etm()

	await Promise.race(mail.map(async message => {
		return Promise.race(message.attachments.map(async attachment => {
			let { content, cid } = attachment
			let where = path.join(directory, cid)
			return writeFile(where, content)
				.then(it => {
					console.log(`saved file: ${where}`)
					console.log(`mime type: ${attachment.contentType}`)
				})
				.catch(exception => {
					console.log(`error with file: ${exception}`)
				})
		}))
	}))

	response.json(mail)
}