1. Email Hippo knowledge base
  2. CORE
  3. Understanding the verification results

What do the CORE email address list verification results mean?

Understand the results returned in your CORE email verification list.

Here are the results returned by our CORE email address list service.

The primary results are Ok, Bad or Unverifiable - this is followed by the additional status.


Ok / Success 

Email address exists and is safe to send.


Bad / Domain is inexistent

The domain of the email address does not exist.

If the email contains a bad domain you can't send an email to it. Check and update the domain, or just delete it from your list.

Bad / Mailbox full

The mailbox of the email address is full and cannot receive any more emails.

There is a mailbox to mail to, which is good news.; but it's full. You could try checking again to see if the recipient has cleared up their inbox. In most cases a full mailbox is seldom checked and just crammed full of rubbish. We wouldn't bother trying to send an email to a full inbox.

Bad / Mail server fault detected

The mail server has been incorrectly configured.

The fault isn't specified, but we know something is wrong with the mail server at the other end. It might be temporary, so it's worth trying again later. A mail server with a permanent fault won't deliver your email to an inbox.

Bad / No MX servers found

No MX servers could be found for the email address.

An MX server is a mail exchange server. The thing that takes mail in and sends it out. If an email domain doesn't have an MX server, it isn't possible to send an email to that address. Delete this from your list, or check the domain for errors and try again.

Bad / Possible spam trap detected

Do not email, this email address is a spam trap.

We think the email address is a spam trap. Don't mail to it. A spam trap is an email address that is monitored but not used. Spam traps can't be opted onto a list, There's a slim chance it might be a really old email address that's been 'recycled' into a spam trap. Mailing to it will probably result in a spam report and may result in your IP address (or your email service provider's IP address) being block listed, which is bad news.

Bad / Server does not support international mailboxes

The server does not accept international  characters for example Chinese characters.

The server we need to check with uses character sets such as Chinese, Arabic or Cyrillic, but it's not set up to cope with international mailboxes. The international language of email is Punycode. We can't speak to international servers that don't translate Punycode. We could try just checking really slowly, shouting at it and pointing but it won't work. Your email address may be OK, but we can't be sure.

Bad / Too many @ signs found

The email address contains more than one @ symbol.

Any email address can only have one of these @ signs. If there are too many, it's an invalid address and you can't mail to it - you'll get a hard bounce. Either check and edit the address or delete it along with your hard bounces. Note: some verification services will have a go at guessing where the right @ should be. Avoid 'guess mail' it's an opt-in nightmare as you can end up mailing the wrong person!


Unverifiable / Domain is well known DEA

This is a disposable email address, do not email.

A DEA is a Disposable Email Address. This email address will last for just a short period of time. People use disposables when they don't want to be reached. You don't want disposables on your mailing list and there's no point sending mail to them. There's a strong link between disposable emails and fraud, so be especially wary of DEAs if you sell online. We scan continually for new disposable email addresses, picking up 99% of them, which is the best detection rate in the business.

Unverifiable / Greylisting

The email server is employing greylisting.

A greylisting result is recorded as 'unverifiable' because we haven't been able to get hold of the mail server to check the status of the email address. Greylisting happens when we try to shake hands with a server and it tells us to come back later and try again. It's usually due to security protocols; by asking us to come back again, the server is checking that we're not spammers. Our advice is to retry the check later as you might get a different response. You can mail to greylisted addresses.

Unverifiable / None

We were unable to retrieve any conclusive information on the email address.

This result is returned when we just don't have any extra information to share. It doesn't happen often, but it's what we say when we are nonplussed. Maybe the server we're trying to reach is on a shut down or there's a system drill going on. Think of it as the mail provider doing things that are ad-hoc and interrupting normal responses.

Unverifiable /  Server is catch all

The server accepts any email address as long as the domain is correct.

Catch-all addresses are like central post boxes. All emails for a domain arrive in one place and get checked and forwarded. A catch all result tells you that your email will get as far as the post room, but it can't tell you whether it will be sent to a recipient.

Unverifiable / Unpredictable system

Mail servers that behave in an unpredictable way giving false results.

The email system used by the mail server is unpredictable. This is a result we use to flag malicious spam traps and systems that are acting strangely and not returning intelligible results to our request.

Unverifiable / Unknown

We were unable to retrieve any conclusive information on the email address.

This is the secondary code that is the exception to prove the rule. We know the primary result is right, but we can't find out more information to explain why.