Email Validation Methods — How mailtester Works

@mailtester/core includes 5 validators that work together to comprehensively validate email addresses in Node.js.

Overview

Validator	Purpose	Speed	Network
Regex	Format validation	Instant	No
Typo	Detect typos	Instant	No
Disposable	Block temp emails	Instant	No
MX	Verify mail servers	~50ms	Yes
SMTP	Verify mailbox	~100ms	Yes

Regex Validator

Validates email format according to RFC 5322 standards.

What it Checks

• Valid email format (local@domain)
• Proper character usage
• Domain structure
• Internationalized domain names (IDN)

Configuration

await validate('user@example.com', {
  validators: {
    regex: { enabled: true }  // Default: true
  }
});

Examples

Email	Result	Reason
user@example.com	Valid	Standard format
user+tag@example.com	Valid	Plus addressing
user.name@example.com	Valid	Dots in local
user@example.co.jp	Valid	IDN support
invalid	Invalid	Missing @ symbol
@example.com	Invalid	Missing local part
user@	Invalid	Missing domain

---

Typo Validator

Detects common domain typos and suggests corrections.

What it Checks

• Common domain misspellings
• TLD typos
• Keyboard proximity errors

Configuration

await validate('user@gmaill.com', {
  validators: {
    typo: { enabled: true }  // Default: true
  }
});

Examples

Input	Suggestion
user@gmaill.com	gmail.com
user@yahooo.com	yahoo.com
user@hotmal.com	hotmail.com
user@outlok.com	outlook.com
user@gogle.com	google.com

Accessing Suggestions

const result = await validate('user@gmaill.com');

if (result.validators.typo?.details?.suggestion) {
  console.log(`Did you mean: ${result.validators.typo.details.suggestion}?`);
  // Output: "Did you mean: gmail.com?"
}

---

Disposable Validator

Blocks temporary / disposable email services used for fake signups.

What it Checks

• ~167,000 known disposable domains via detect-disposable-email (exact match)
• 399 wildcard base domains (any subdomain of a base is disposable, e.g. x.y.10mail.org)
• Pattern-based heuristics (tempmail*, 10minutemail*, throwaway*, …)
• Custom blacklist / whitelist you configure

Data source (v1.2.0+)

Item	Detail
Package	detect-disposable-email
Replaces	Unmaintained disposable-email-domains
Matching	Exact + wildcard parent walk + IDN/punycode
Updates	Data refreshed upstream; mailtester depends on ^1.1.0

Configuration

await validate('user@mailinator.com', {
  validators: {
    disposable: {
      enabled: true,                    // Default: true
      customBlacklist: ['spam.example'], // Always treat as disposable
      customWhitelist: ['partner.com'],  // Always allow (even if in list)
      enablePatternDetection: true       // Default: true
    }
  }
});

Option	Type	Default	Description
enabled	boolean	true	Run disposable checks
customBlacklist	string[]	[]	Domains always blocked
customWhitelist	string[]	[]	Domains always allowed (overrides list + patterns)
enablePatternDetection	boolean	true	Heuristic name patterns

Priority order: whitelist → blacklist → known list (exact/wildcard) → patterns.

Blocked Services (Examples)

• mailinator.com
• guerrillamail.com
• 10minutemail.com
• tempmail.com
• throwaway.email
• Wildcard subdomains such as anything.10mail.org
• And 160,000+ more...

Example

const result = await validate('test@mailinator.com');

console.log(result.valid);   // false
console.log(result.reason);  // "disposable"
console.log(result.validators.disposable?.error?.details);
// { domain: 'mailinator.com', reason: 'known_disposable' }

False positives

If a legitimate domain is blocked, allow it locally:

await validate('user@legit-corp.com', {
  validators: {
    disposable: {
      enabled: true,
      customWhitelist: ['legit-corp.com']
    }
  }
});

Or open an issue on detect-disposable-email for a data fix.

---

MX Validator

Verifies domain has valid mail exchange (MX) servers.

What it Checks

• MX records exist
• DNS resolution works
• Fallback to A records if no MX

Configuration

await validate('user@example.com', {
  validators: {
    mx: { enabled: true }  // Default: true
  }
});

How it Works

1. Queries DNS for MX records
2. If no MX found, checks for A records (fallback)
3. Validates at least one mail server exists

Example

const result = await validate('user@nonexistent-domain-xyz.com');

if (!result.validators.mx?.valid) {
  console.log('Domain has no mail servers');
}

Performance

MX validation requires DNS lookup (~50ms). For faster validation, use the balanced or permissive preset.

---

SMTP Validator

Verifies mailbox exists by connecting to the mail server.

What it Checks

• SMTP connection to port 25
• HELO/EHLO handshake
• MAIL FROM command
• RCPT TO verification
• Mailbox existence

Configuration

await validate('user@example.com', {
  validators: {
    smtp: { enabled: true }  // Default: true
  }
});

How it Works

1. Resolves MX records for domain
2. Connects to mail server on port 25
3. Performs SMTP handshake
4. Sends RCPT TO command to verify mailbox
5. Checks response code

Example

const result = await validate('nonexistent-user@gmail.com');

if (!result.validators.smtp?.valid) {
  console.log('Mailbox does not exist');
}

Limitations

• Some mail servers block SMTP verification
• Corporate firewalls may block port 25
• Gmail and other providers may rate limit

Consider using balanced preset if you experience timeouts.

---

Validation Order

Validators run in this order:

1. Regex - Quick format check (stops if invalid)
2. Typo - Check for domain typos
3. Disposable - Check if disposable
4. MX - Verify mail servers exist
5. SMTP - Verify mailbox exists

With earlyExit: true (default), validation stops at the first failure.

---

Disabling Validators

Disable validators you don't need:

// Fast validation (no network calls)
await validate('user@example.com', {
  validators: {
    regex: { enabled: true },
    typo: { enabled: true },
    disposable: { enabled: true },
    mx: { enabled: false },
    smtp: { enabled: false }
  }
});

Or use a preset:

// Balanced: No SMTP
await validate('user@example.com', { preset: 'balanced' });

// Permissive: Regex only
await validate('user@example.com', { preset: 'permissive' });