Plus-addressing normalization

Gmail and many providers route user+tag@gmail.com to the same mailbox as user@gmail.com. Without stripping plus-tags before deduplication, a single subscriber can appear multiple times with different tags (user+promo@, user+news@, user+test@) — each receiving its own copy of every send. The duplication inflates list counts, skews per-contact engagement metrics, and in markets with per-contact billing (most ESPs), directly increases cost. More critically, a suppression or unsubscribe recorded against the base address won't suppress the tagged variants, creating compliance exposure under CAN-SPAM § 5's unsubscribe-honoring requirement.

Strip the plus-tag from the local part during canonicalization before the deduplication lookup:

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

const canonical = canonicalizeEmail(rawEmail)
await db.contact.upsert({
  where: { email: canonical },
  create: { email: canonical, email_original: rawEmail },
  update: { updated_at: new Date() }
})

Store both email (canonical) and email_original (as submitted). Suppressions and unsubscribes must be keyed on the canonical form so all variants are covered. Note: Outlook's plus-addressing behavior differs from Gmail — plus-stripping is safe for @gmail.com and @yahoo.com domains; apply universally with awareness that it is a dedup heuristic, not a routing guarantee.

• ID: data-quality-list-hygiene.dedup-normalization.plus-address-normalization

• Severity: low

• What to look for: Enumerate all email normalization steps applied before deduplication. Check whether the system strips the plus-tag from email addresses before deduplication checks. alice+promo@gmail.com and alice@gmail.com refer to the same mailbox. Without stripping the tag, a user can subscribe with multiple plus-tagged variants and receive duplicate sends. Look for regex or string manipulation that removes +... from the local-part before lookup.

• Pass criteria: Plus-tags are stripped from the local-part for deduplication purposes. Optionally: the canonical (stripped) form is stored while the original is kept in a separate column for reference. At least 1 normalization function handles plus-tag stripping.

• Fail criteria: Plus-tagged addresses are treated as unique addresses. Multiple +tag variants for the same base address can co-exist in the list.

• Skip (N/A) when: The system explicitly uses plus-tagging for contact attribution and intentionally tracks per-tag variants.

• Detail on fail: Example: "user+campaign1@gmail.com and user+campaign2@gmail.com treated as separate contacts — same person receives duplicate sends"

• Remediation: Strip plus-tags before deduplication lookup:

  function canonicalizeEmail(email: string): string {
    const [local, domain] = email.toLowerCase().trim().split('@')
    if (!local || !domain) return email.toLowerCase().trim()
    // Strip plus-tag from local part
    const canonicalLocal = local.split('+')[0]
    return `${canonicalLocal}@${domain}`
  }
  
  // Store canonical form; optionally keep original:
  const canonical = canonicalizeEmail(rawEmail)
  await db.contact.upsert({
    where: { email: canonical },
    create: { email: canonical, email_original: rawEmail, ... },
    update: { updated_at: new Date() }
  })
  

  Note: Gmail and many providers route user+tag@gmail.com to user@gmail.com. However, Outlook's plus-addressing behavior differs — apply this normalization selectively or universally with awareness of false deduplication risk on providers that treat + as a literal character (rare in practice).