What the checker looks for
- Invalid syntax. The address must have one @, a local part of up to 64 characters without spaces or double dots, and a domain with at least one dot and a real-looking ending. Quoted local parts and IP addresses as domains are rare in sign-up lists and are flagged as invalid.
- Exact and case duplicates. The same address twice, or the same address with different capital letters, such as Anna@Example.com and anna@example.com. The first one is kept.
- Gmail variants. Gmail ignores dots in the part before the @ and anything after a plus sign, so a.nna+win@gmail.com reaches the same inbox as anna@gmail.com. The checker only applies this rule to gmail.com and googlemail.com addresses, because other providers treat dots and plus signs differently.
- Likely domain typos. Domains such as gmial.com, hotmial.com or yaho.com are compared with a list of common mailbox domains, including big Nordic and European ones like hotmail.dk, live.dk, gmx.de, web.de, t-online.de, orange.fr, libero.it, wp.pl and seznam.cz. A domain that is one or two letters away from one of them, or that ends in .con or .cmo, gets a suggested fix. It is a suggestion: check it before you change an address.
- Disposable domains. Addresses from throwaway inbox services, matched against a public list of known disposable domains.
- Role addresses. Addresses such as info@, sales@, support@ and noreply@ belong to a team or a system rather than a person, so it is often unclear who agreed to receive your emails.
What it does not check
The checker looks at the addresses themselves. It does not contact mail servers, so it cannot tell you whether a mailbox exists or still works. A correct-looking address can still bounce, and only a confirmation email proves that someone owns it. That is why many senders use double opt-in.
Why it matters
Duplicates make you send the same email twice and count one person as two. Typos and invalid addresses bounce, and a high bounce rate can hurt how your emails are delivered. Disposable addresses tend to belong to people who wanted the prize but not the newsletter. If you run competitions, read how to get better lead quality from competitions, and keep your sign-up form short so people type their real address.
Stop bad addresses before they reach the list
Cleaning a list afterwards works, but it is easier when bad sign-ups never get in. Scratcher campaigns block throwaway email addresses at sign-up and stop people entering twice. See how to run an online competition. Need to draw winners from your cleaned list? Use the random winner picker.
