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.