fix(smtp): skip the reverse DNS lookup of connecting clients (#104)

smtp-server reverse resolves every connecting client before it sends the
220 greeting, unless disableReverseLookup is set. MailDev never reads the
resolved hostname, and without a network the lookup does not answer, so
each connection waited for smtp-server to give up on it.

packages/smtp/src/__tests__/reverse-lookup.test.ts

@@ -0,0 +1,49 @@
+import { describe, it, expect, beforeEach, afterEach, vi } from 'vitest'
+import dns from 'node:dns'
+import { mkdtemp, rm } from 'node:fs/promises'
+import { tmpdir } from 'node:os'
+import { join } from 'node:path'
+import nodemailer from 'nodemailer'
+import { MemoryStorage } from '@maildev/core'
+import { SMTPServer } from '../server.js'
+
+describe('client reverse DNS lookup', () => {
+  let mailDir: string
+  let storage: MemoryStorage
+  let server: SMTPServer
+
+  beforeEach(async () => {
+    mailDir = await mkdtemp(join(tmpdir(), 'maildev-reverse-'))
+    storage = new MemoryStorage()
+    await storage.initialize()
+    server = new SMTPServer({ storage, mailDir, port: 0, host: '127.0.0.1' })
+    await server.start()
+  })
+
+  afterEach(async () => {
+    vi.restoreAllMocks()
+    await server.stop()
+    await rm(mailDir, { recursive: true, force: true })
+  })
+
+  // Without a network the lookup never answers, and smtp-server holds back the
+  // 220 greeting until it gives up on it (#104).
+  it('accepts mail without reverse resolving the connecting client', async () => {
+    const reverse = vi.spyOn(dns, 'reverse').mockImplementation(() => {})
+
+    const transport = nodemailer.createTransport({
+      host: '127.0.0.1',
+      port: server.getPort(),
+      secure: false,
+    })
+    await transport.sendMail({
+      from: 'a@example.com',
+      to: 'b@example.com',
+      subject: 'offline',
+      text: 'hello',
+    })
+
+    expect(await storage.getAll()).toHaveLength(1)
+    expect(reverse).not.toHaveBeenCalled()
+  })
+})

packages/smtp/src/server.ts

@@ -524,6 +524,9 @@ export class SMTPServer extends EventEmitter {
       logger: false,
       hideSTARTTLS: true,
       disabledCommands: ['AUTH'] as string[],
+      // The client hostname is never used, and without a network the lookup
+      // stalls every connection before the greeting (#104).
+      disableReverseLookup: true,
     }
 
     // Advertise and enforce a maximum message size. smtp-server rejects