diff --git a/packages/cli/src/__tests__/config/env.test.ts b/packages/cli/src/__tests__/config/env.test.ts
new file mode 100644
index 0000000..95b048e
--- /dev/null
+++ b/packages/cli/src/__tests__/config/env.test.ts
@@ -0,0 +1,204 @@
+import { describe, it, expect, beforeEach, afterEach } from 'vitest'
+import { loadEnvConfig, getEnvVarName } from '../../config/env.js'
+
+describe('loadEnvConfig', () => {
+  const originalEnv = process.env
+
+  beforeEach(() => {
+    // Reset process.env before each test
+    process.env = { ...originalEnv }
+  })
+
+  afterEach(() => {
+    process.env = originalEnv
+  })
+
+  it('should return empty config when no env vars set', () => {
+    // Clear relevant env vars
+    delete process.env.MAILDEV_SMTP_PORT
+    delete process.env.MAILDEV_WEB_PORT
+    delete process.env.MAILDEV_IP
+
+    const config = loadEnvConfig()
+    expect(config.smtp).toBeUndefined()
+    expect(config.web).toBeUndefined()
+  })
+
+  it('should parse MAILDEV_SMTP_PORT as number', () => {
+    process.env.MAILDEV_SMTP_PORT = '2025'
+
+    const config = loadEnvConfig()
+    expect(config.smtp).toBe(2025)
+    expect(typeof config.smtp).toBe('number')
+  })
+
+  it('should parse MAILDEV_WEB_PORT as number', () => {
+    process.env.MAILDEV_WEB_PORT = '3080'
+
+    const config = loadEnvConfig()
+    expect(config.web).toBe(3080)
+  })
+
+  it('should parse MAILDEV_MAX_MESSAGE_SIZE as number', () => {
+    process.env.MAILDEV_MAX_MESSAGE_SIZE = '1048576'
+
+    const config = loadEnvConfig()
+    expect(config.maxMessageSize).toBe(1048576)
+    expect(typeof config.maxMessageSize).toBe('number')
+  })
+
+  it('should load MAILDEV_IP as string', () => {
+    process.env.MAILDEV_IP = '127.0.0.1'
+
+    const config = loadEnvConfig()
+    expect(config.ip).toBe('127.0.0.1')
+  })
+
+  it('should load MAILDEV_WEB_IP as string', () => {
+    process.env.MAILDEV_WEB_IP = '192.168.1.1'
+
+    const config = loadEnvConfig()
+    expect(config.webIp).toBe('192.168.1.1')
+  })
+
+  it('should load authentication env vars', () => {
+    process.env.MAILDEV_INCOMING_USER = 'smtp_user'
+    process.env.MAILDEV_INCOMING_PASS = 'smtp_pass'
+    process.env.MAILDEV_WEB_USER = 'web_user'
+    process.env.MAILDEV_WEB_PASS = 'web_pass'
+
+    const config = loadEnvConfig()
+    expect(config.incomingUser).toBe('smtp_user')
+    expect(config.incomingPass).toBe('smtp_pass')
+    expect(config.webUser).toBe('web_user')
+    expect(config.webPass).toBe('web_pass')
+  })
+
+  it('should load outgoing relay env vars', () => {
+    process.env.MAILDEV_OUTGOING_HOST = 'smtp.example.com'
+    process.env.MAILDEV_OUTGOING_PORT = '587'
+    process.env.MAILDEV_OUTGOING_USER = 'relay_user'
+    process.env.MAILDEV_OUTGOING_PASS = 'relay_pass'
+
+    const config = loadEnvConfig()
+    expect(config.outgoingHost).toBe('smtp.example.com')
+    expect(config.outgoingPort).toBe(587)
+    expect(config.outgoingUser).toBe('relay_user')
+    expect(config.outgoingPass).toBe('relay_pass')
+  })
+
+  it('should load MAILDEV_MAIL_DIRECTORY', () => {
+    process.env.MAILDEV_MAIL_DIRECTORY = '/var/mail'
+
+    const config = loadEnvConfig()
+    expect(config.mailDirectory).toBe('/var/mail')
+  })
+
+  it('should load MAILDEV_BASE_PATHNAME', () => {
+    process.env.MAILDEV_BASE_PATHNAME = '/mail/'
+
+    const config = loadEnvConfig()
+    expect(config.basePathname).toBe('/mail/')
+  })
+
+  it('should ignore invalid port numbers', () => {
+    process.env.MAILDEV_SMTP_PORT = 'not-a-number'
+
+    const config = loadEnvConfig()
+    expect(config.smtp).toBeUndefined()
+  })
+
+  it('should handle multiple env vars simultaneously', () => {
+    process.env.MAILDEV_SMTP_PORT = '2025'
+    process.env.MAILDEV_WEB_PORT = '3080'
+    process.env.MAILDEV_IP = '0.0.0.0'
+    process.env.MAILDEV_MAIL_DIRECTORY = '/tmp/mail'
+
+    const config = loadEnvConfig()
+    expect(config.smtp).toBe(2025)
+    expect(config.web).toBe(3080)
+    expect(config.ip).toBe('0.0.0.0')
+    expect(config.mailDirectory).toBe('/tmp/mail')
+  })
+
+  it('should parse MAILDEV_AUTO_RELAY=true as boolean true', () => {
+    process.env.MAILDEV_AUTO_RELAY = 'true'
+
+    const config = loadEnvConfig()
+    expect(config.autoRelay).toBe(true)
+  })
+
+  it('should parse MAILDEV_AUTO_RELAY=1 as boolean true', () => {
+    process.env.MAILDEV_AUTO_RELAY = '1'
+
+    const config = loadEnvConfig()
+    expect(config.autoRelay).toBe(true)
+  })
+
+  it('should parse present-but-empty MAILDEV_AUTO_RELAY as boolean true', () => {
+    process.env.MAILDEV_AUTO_RELAY = ''
+
+    const config = loadEnvConfig()
+    expect(config.autoRelay).toBe(true)
+  })
+
+  it('should parse MAILDEV_AUTO_RELAY=false as boolean false', () => {
+    process.env.MAILDEV_AUTO_RELAY = 'false'
+
+    const config = loadEnvConfig()
+    expect(config.autoRelay).toBe(false)
+  })
+
+  it('should parse MAILDEV_AUTO_RELAY=0 as boolean false', () => {
+    process.env.MAILDEV_AUTO_RELAY = '0'
+
+    const config = loadEnvConfig()
+    expect(config.autoRelay).toBe(false)
+  })
+
+  it('should parse MAILDEV_AUTO_RELAY as a recipient email', () => {
+    process.env.MAILDEV_AUTO_RELAY = 'qa@example.com'
+
+    const config = loadEnvConfig()
+    expect(config.autoRelay).toBe('qa@example.com')
+  })
+
+  it('should load MAILDEV_AUTO_RELAY_RULES as a path string', () => {
+    process.env.MAILDEV_AUTO_RELAY_RULES = '/config/rules.json'
+
+    const config = loadEnvConfig()
+    expect(config.autoRelayRules).toBe('/config/rules.json')
+  })
+
+  it('should leave autoRelay and autoRelayRules unset when env vars are absent', () => {
+    delete process.env.MAILDEV_AUTO_RELAY
+    delete process.env.MAILDEV_AUTO_RELAY_RULES
+
+    const config = loadEnvConfig()
+    expect(config.autoRelay).toBeUndefined()
+    expect(config.autoRelayRules).toBeUndefined()
+  })
+})
+
+describe('getEnvVarName', () => {
+  it('should return env var name for smtp', () => {
+    expect(getEnvVarName('smtp')).toBe('MAILDEV_SMTP_PORT')
+  })
+
+  it('should return env var name for web', () => {
+    expect(getEnvVarName('web')).toBe('MAILDEV_WEB_PORT')
+  })
+
+  it('should return env var name for ip', () => {
+    expect(getEnvVarName('ip')).toBe('MAILDEV_IP')
+  })
+
+  it('should return env var name for mailDirectory', () => {
+    expect(getEnvVarName('mailDirectory')).toBe('MAILDEV_MAIL_DIRECTORY')
+  })
+
+  it('should return null for unmapped config keys', () => {
+    expect(getEnvVarName('mcp')).toBeNull()
+    expect(getEnvVarName('verbose')).toBeNull()
+  })
+})
diff --git a/packages/cli/src/config/env.ts b/packages/cli/src/config/env.ts
new file mode 100644
index 0000000..404d0a5
--- /dev/null
+++ b/packages/cli/src/config/env.ts
@@ -0,0 +1,166 @@
+/**
+ * MailDev Environment Variable Configuration
+ *
+ * Maps MAILDEV_* environment variables to configuration options
+ */
+
+import type { PartialMailDevConfig } from './types.js'
+
+/**
+ * Environment variable to config key mapping
+ */
+const ENV_MAPPING: Record<string, keyof PartialMailDevConfig> = {
+  // SMTP
+  MAILDEV_SMTP_PORT: 'smtp',
+  MAILDEV_IP: 'ip',
+  MAILDEV_INCOMING_USER: 'incomingUser',
+  MAILDEV_INCOMING_PASS: 'incomingPass',
+  MAILDEV_MAX_MESSAGE_SIZE: 'maxMessageSize',
+
+  // Web/API
+  MAILDEV_WEB_PORT: 'web',
+  MAILDEV_WEB_IP: 'webIp',
+  MAILDEV_WEB_USER: 'webUser',
+  MAILDEV_WEB_PASS: 'webPass',
+  MAILDEV_BASE_PATHNAME: 'basePathname',
+  MAILDEV_HTTPS: 'https',
+  MAILDEV_HTTPS_CERT: 'httpsCert',
+  MAILDEV_HTTPS_KEY: 'httpsKey',
+
+  // Outgoing/Relay
+  MAILDEV_OUTGOING_HOST: 'outgoingHost',
+  MAILDEV_OUTGOING_PORT: 'outgoingPort',
+  MAILDEV_OUTGOING_USER: 'outgoingUser',
+  MAILDEV_OUTGOING_PASS: 'outgoingPass',
+  MAILDEV_AUTO_RELAY: 'autoRelay',
+  MAILDEV_AUTO_RELAY_RULES: 'autoRelayRules',
+
+  // Storage
+  MAILDEV_MAIL_DIRECTORY: 'mailDirectory',
+  MAILDEV_MAX_EMAILS: 'maxEmails',
+
+  // API URL (for MCP client mode)
+  MAILDEV_API_URL: 'webIp', // Special handling needed
+}
+
+/**
+ * Environment variables that should be parsed as numbers
+ */
+const NUMBER_VARS = new Set([
+  'MAILDEV_SMTP_PORT',
+  'MAILDEV_WEB_PORT',
+  'MAILDEV_OUTGOING_PORT',
+  'MAILDEV_MAX_MESSAGE_SIZE',
+  'MAILDEV_MAX_EMAILS',
+])
+
+/**
+ * Environment variables that should be parsed as booleans
+ */
+const BOOLEAN_VARS = new Set([
+  'MAILDEV_INCOMING_SECURE',
+  'MAILDEV_OUTGOING_SECURE',
+  'MAILDEV_HTTPS',
+  'MAILDEV_DISABLE_WEB',
+  'MAILDEV_MCP',
+  'MAILDEV_VERBOSE',
+  'MAILDEV_SILENT',
+])
+
+/**
+ * Parse boolean environment variable
+ */
+function parseBoolean(value: string): boolean {
+  return value.toLowerCase() === 'true' || value === '1'
+}
+
+/**
+ * MAILDEV_AUTO_RELAY is boolean | string: true/1/empty enable relay to
+ * each mail's own recipients; false/0 disable; any other value is the
+ * override recipient.
+ * Do not put this in BOOLEAN_VARS — that would parse an email as false.
+ */
+function parseAutoRelay(value: string): boolean | string {
+  if (value === '' || value.toLowerCase() === 'true' || value === '1') {
+    return true
+  }
+  if (value.toLowerCase() === 'false' || value === '0') {
+    return false
+  }
+  return value
+}
+
+/**
+ * Load configuration from environment variables
+ *
+ * @returns Partial configuration from environment
+ */
+export function loadEnvConfig(): PartialMailDevConfig {
+  const config: PartialMailDevConfig = {}
+
+  for (const [envVar, configKey] of Object.entries(ENV_MAPPING)) {
+    const value = process.env[envVar]
+    if (value === undefined) continue
+
+    if (envVar === 'MAILDEV_AUTO_RELAY') {
+      config.autoRelay = parseAutoRelay(value)
+    } else if (NUMBER_VARS.has(envVar)) {
+      const num = parseInt(value, 10)
+      if (!isNaN(num)) {
+        ;(config as Record<string, unknown>)[configKey] = num
+      }
+    } else {
+      ;(config as Record<string, unknown>)[configKey] = value
+    }
+  }
+
+  // Handle boolean environment variables
+  for (const envVar of BOOLEAN_VARS) {
+    const value = process.env[envVar]
+    if (value !== undefined) {
+      const key = envVarToConfigKey(envVar)
+      if (key) {
+        ;(config as Record<string, unknown>)[key] = parseBoolean(value)
+      }
+    }
+  }
+
+  return config
+}
+
+/**
+ * Convert environment variable name to config key
+ */
+function envVarToConfigKey(envVar: string): keyof PartialMailDevConfig | null {
+  // Remove MAILDEV_ prefix and convert to camelCase
+  const withoutPrefix = envVar.replace(/^MAILDEV_/, '')
+  const parts = withoutPrefix.toLowerCase().split('_')
+  const camelCase = parts
+    .map((part, i) => (i === 0 ? part : part.charAt(0).toUpperCase() + part.slice(1)))
+    .join('')
+
+  // Map specific env vars to config keys
+  const mapping: Record<string, keyof PartialMailDevConfig> = {
+    smtpPort: 'smtp',
+    webPort: 'web',
+    incomingSecure: 'incomingSecure',
+    outgoingSecure: 'outgoingSecure',
+    disableWeb: 'disableWeb',
+    verbose: 'verbose',
+    silent: 'silent',
+    mcp: 'mcp',
+  }
+
+  return mapping[camelCase] || null
+}
+
+/**
+ * Get environment variable name for a config key
+ * (useful for error messages and documentation)
+ */
+export function getEnvVarName(configKey: keyof PartialMailDevConfig): string | null {
+  for (const [envVar, key] of Object.entries(ENV_MAPPING)) {
+    if (key === configKey) return envVar
+  }
+  return null
+}