Email Normalizer

If you've ever imported a customer list and watched it report "1,247 unique addresses" when the source had 1,300 entries, you've already met the email-normalisation problem. The same human is often represented by half a dozen syntactic variants of the same address. Lowercasing helps. Stripping plus-aliases helps more. Knowing the Gmail-specific dot rule — which is the single biggest source of duplicate counts in any consumer mailing list — closes most of the remaining gap.

The Gmail dot rule, explained

Gmail treats dots in the local part of @gmail.com and @googlemail.com addresses as cosmetic — but not on Google Workspace work or school domains, where Google says dots do change the address (Gmail Help). The addresses [email protected], [email protected], and [email protected] all deliver to the same inbox. This is documented Gmail behaviour, not a bug — it was originally designed so users could "format" their email for readability without losing reach. The side effect is that signup forms and CRMs see the same person as different people. Stripping all dots from the local part of a @gmail.com or @googlemail.com address is the single most-effective deduplication rule you can apply.

The +tag alias

Sub-addressing — the trick of writing [email protected] to get mail still delivered to [email protected] — is described for mail filtering in RFC 5233 (2008), and many large providers accept it — Microsoft turns it on by default in Exchange Online. Support varies by provider, so check before relying on it. Users do this deliberately, often to filter incoming mail ([email protected] for RECATOOLS signups, [email protected] for Amazon receipts). For deduplication, you almost always want to strip the +suffix and treat all variants as the same person. The exception is fraud detection — repeated +1, +2, +3 variants on a free-trial endpoint are usually one person abusing the limit, and the +tag is the giveaway.

Disposable email detection

Mailinator, 10minutemail, guerrillamail, tempmail, and their friends are throwaway-mail services — they hand out an email address that auto-expires in minutes. They're great for users who don't want to give their real address to a one-time signup; they're a problem for any business model where re-engaging a user matters. The tool here ships with a curated list of 85 known disposable-email domains. The list is not exhaustive (new domains spin up weekly), but it catches the common offenders. For production systems with real fraud exposure, use a maintained disposable-domain list or a verification service on top of this client-side check.

Role accounts and ASEAN gotchas

Role accounts (info@, admin@, support@, billing@, noreply@) are not individual humans — they're inboxes shared by a team or function. They are valid email addresses, but for marketing-list use they typically should be filtered out separately. The tool flags them so you can decide. Another regional gotcha: many Southeast Asian companies use addresses on country-specific subdomains ([email protected], [email protected]) — these are not "weird," they're standard, and the normaliser treats them identically to .com addresses. Don't strip the .sg or .my; it's part of the canonical address.

What this is not

Normalisation is not verification. The output is "this is the canonical spelling of an email that looks valid" — not "this email exists and accepts mail." Verifying that requires actually delivering to the address (or talking to the receiving SMTP server), which can't happen client-side and isn't this tool's job. Pair normalisation with a real-time verification API at the moment of signup if your business model depends on deliverability.