# Domain alias resolution

- **Pattern:** `ab-000861` (`data-quality-list-hygiene.dedup-normalization.domain-alias-resolution`)
- **Severity:** low
- **Lifecycle:** active
- **Last modified:** 2026-04-18
- **Canonical URL:** https://auditbuffet.com/patterns/ab-000861
- **License:** CC-BY-4.0 — attribute to AuditBuffet Pattern Catalog (https://auditbuffet.com/patterns/ab-000861)

## Why it matters

In certain regions, Google operated `googlemail.com` as an alias for `gmail.com` — `alice@googlemail.com` and `alice@gmail.com` are the same mailbox. Without alias resolution, the same person can appear as two separate contacts, receiving duplicate sends and having their unsubscribe applied only to the alias variant. While `googlemail.com` is the most common example, additional alias relationships exist for other providers in specific markets. Duplicate contacts from unresolved aliases inflate list metrics, add send cost, and introduce suppression gaps when the unsubscribe is captured under the aliased domain.

## Severity rationale

Low because domain alias collisions are infrequent and affect a narrow set of known providers, creating manageable duplication rather than systematic data corruption.

## Remediation

Apply a domain alias map during normalization, resolving aliased domains to their canonical form before deduplication lookup:

```ts
const DOMAIN_ALIASES: Record<string, string> = {
  'googlemail.com': 'gmail.com',
  'googlemail.co.uk': 'gmail.com',
}

function normalizeEmail(email: string): string {
  const [local, domain] = email.toLowerCase().trim().split('@')
  if (!local || !domain) return email.toLowerCase().trim()
  const canonicalLocal = local.split('+')[0]
  const canonicalDomain = DOMAIN_ALIASES[domain] ?? domain
  return `${canonicalLocal}@${canonicalDomain}`
}
```

Combine this with plus-tag stripping (see `ab-000860`) in a single `normalizeEmail` function so all normalization logic runs in one pass. Add new alias entries as you discover them via duplicate detection reports.

## Detection

- **ID:** `domain-alias-resolution`
- **Severity:** `low`
- **What to look for:** Count all domain alias mappings defined in the normalization logic. Check whether the system resolves known domain aliases before storage. The most common example: `googlemail.com` is an alias of `gmail.com` in some regions. `alice@googlemail.com` and `alice@gmail.com` are the same mailbox. Look for a domain alias map applied during normalization.
- **Pass criteria:** Known domain aliases are resolved to canonical domains before deduplication with at least 1 alias mapping. At minimum, `googlemail.com` → `gmail.com` is handled.
- **Fail criteria:** Domain aliases are not resolved, allowing the same person to appear as duplicates with aliased domains.
- **Skip (N/A) when:** The system serves a narrow, known audience (e.g., B2B enterprise only) where personal email domain aliases are not expected.
- **Detail on fail:** Example: `"alice@googlemail.com and alice@gmail.com stored as separate contacts"`
- **Remediation:** Apply a domain alias map during normalization:

  ```ts
  const DOMAIN_ALIASES: Record<string, string> = {
    'googlemail.com': 'gmail.com',
    'googlemail.co.uk': 'gmail.com',
    // Add others as discovered
  }

  function resolveDomain(domain: string): string {
    return DOMAIN_ALIASES[domain.toLowerCase()] ?? domain.toLowerCase()
  }

  function normalizeEmail(email: string): string {
    const [local, domain] = email.toLowerCase().trim().split('@')
    if (!local || !domain) return email.toLowerCase().trim()
    const canonicalLocal = local.split('+')[0]
    return `${canonicalLocal}@${resolveDomain(domain)}`
  }
  ```

## External references

- iso-25010 functional-correctness

Taxons: data-integrity

HTML version: https://auditbuffet.com/patterns/ab-000861