Writing a Gmail Bot: OAuth Client Secret or Service Account?

I want to write a bot replying to some income messages of my gmail inbox. It would be nice if I can use it with an address with a plus sign: "myname+foo@gmail.com"

1. What type of credentials do I need: OAuth with client secret or service account? Currently I try my way with a service account.
2. I try to manage my email from a service account this way:

'use strict';

const Google = require('googleapis');
const Gmail = Google.gmail('v1');

const key = require('./secrets/file-from-service-account.json');

const jwtClient = new Google.auth.JWT(
  key.client_email,
  null,
  key.private_key,
  [
    'https://mail.google.com/',
    'https://www.googleapis.com/auth/gmail.readonly',
  ],
  'myemail@gmail.com' // Not a service account email, but my gmail account.
);

jwtClient.authorize(function (err, tokens) {
  if (err) {
    console.log(err);
    return;
  }

  Gmail.users.messages.list({
    userId: 'myemail@gmail.com',
    auth: jwtClient,
  }, (err, response) => {

    if (err) {
      console.log('The API returned an error:', err)
      return;
    }
    console.log('RESPONSE', response);

  });

});

Here is the output:

$ node index.js                                                                                                                                                                     
Error: unauthorized_client
    at Request._callback (/home/ilyaigpetrov/Repos/ac-gmail-bot/node_modules/gtoken/lib/index.js:215:34)
    at Request.self.callback (/home/ilyaigpetrov/Repos/ac-gmail-bot/node_modules/request/request.js:188:22)
    at emitTwo (events.js:106:13)
    at Request.emit (events.js:191:7)
    at Request.<anonymous> (/home/ilyaigpetrov/Repos/ac-gmail-bot/node_modules/request/request.js:1171:10)
    at emitOne (events.js:96:13)
    at Request.emit (events.js:188:7)
    at IncomingMessage.<anonymous> (/home/ilyaigpetrov/Repos/ac-gmail-bot/node_modules/request/request.js:1091:12)
    at IncomingMessage.g (events.js:292:16)
    at emitNone (events.js:91:20)

Why does this error happen? Do I need to give Service Account a permission to use myemail@gmail.com somehow?

via ilyaigpetrov