Our service communicates with mail servers to determine whether an email exists or not.

Occasionally, our services will return a status of 'Unverifiable' with a reason code of 'None' in response to a query on an email address.

Why This Happens

Possible explanations for this combination of codes are:

  • Target MX record uses Office365 or;
  • Mail provider implementing custom mailbox shutdowns

Why This Affects Office365

Email Hippo deep mailbox inspections are designed to report email addresses in one of four primary response codes:

  • Ok
  • Bad
  • RetryLater
  • Unverifiable

For the Unverifiable status, there are many reason codes that can be found here.

For the 'ServerIsCatchAll' status, Email Hippo correctly identifies Office365 services that are configured to report as 'CatchAll'. However, for reasons only known to Microsoft, Office365 services reporting as 'CatchAll' are actually not 'CatchAll' and can / do return email hard bounces if a customer were to send an email to these addresses. For this reason, we do not report 'CatchAll' addresses as far as Office365 services are concerned. Instead, we report reason code of 'None' for Office365 'CatchAll' addresses.


1. Can Email Hippo do any Office365 addresses?
Yes. We report on servers that are not configured as CatchAll. This means that customers can expect to receive valid 'Ok' and 'Bad' results for Office365 hosted domains that are not configured as 'CatchAll'
2. Is it only Office365 that behaves in this way?
No but it is the most significant (in terms of global email volume) service that does so.
3. What about other possibilities for this behavior?
SMTP forward only, relay or 'SmartHosts' can respond in this fashion.
4. What proportion of services behave in this way?
Other than Office365, and based on our historic analysis, less than 2% of 'CatchAll' services will result in hard bounces if a customer chooses to send email to 'CatchAll'.
5. Can I identify Office365 domains from Email Hippo?
Yes, but only on API service endpoints greater than 2.0. Please contact us below for details of endpoint versions that are appropriate for your use case.
6. Generally, does Email Hippo recommend sending emails to 'CatchAll' email addresses.
Generally, no. Email Hippo recommends sending emails only to addresses that we have identified as 'Ok'. However, there are situations where 'CatchAll' addresses are simply too valuable to discard. In this situation, we suggest following our guidelines that are described here.

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