A relay sends; an agent also has to read the mail that comes back.

// Outbound is easy — SMTP2GO is a drop-in relay.
import nodemailer from "nodemailer";
const relay = nodemailer.createTransport({
  host: "mail.smtp2go.com", port: 587,
  auth: { user: process.env.SMTP2GO_USER, pass: process.env.SMTP2GO_PASS },
});
await relay.sendMail({ from: "agent@myapp.ai", to, subject, html });

// Inbound has no SMTP2GO path. Stand up a SECOND mailbox and poll it:
import { ImapFlow } from "imapflow";
import { simpleParser } from "mailparser";
const imap = new ImapFlow({
  host: "imap.fastmail.com", port: 993, secure: true,
  auth: { user: process.env.IMAP_USER, pass: process.env.IMAP_PASS },
});
await imap.connect();
for await (const msg of imap.fetch({ seen: false }, { source: true })) {
  const mail = await simpleParser(msg.source);   // MIME parsing is yours
  // SPF / DKIM / DMARC? re-derive them yourself before trusting the sender.
  await runAgent({ task: mail.text, from: mail.from?.text });
}

POST /your-webhook   Content-Type: application/json
x-mailkite-signature: t=…,v1=…   (HMAC-SHA256 — verify locally)

{
  "id": "msg_2Hk9…",
  "type": "email.received",
  "from": { "address": "ada@example.com" },
  "to": [{ "address": "support@myapp.ai" }],
  "subject": "Re: invoice #1042",
  "text": "Looks good — approved!",
  "html": "<p>Looks good — approved!</p>",
  "threadId": "<a1b2c3@mail.example.com>",
  "auth": { "spf": "pass", "dkim": "pass", "dmarc": "pass", "spam": "ham" },
  "attachments": [
    { "filename": "po.pdf", "contentType": "application/pdf",
      "size": 18213, "url": "https://api.mailkite.dev/att/2Hk9…/0?sig=…" }
  ]
}