Sometimes conclusive verification result cannot be achieved due to the mail server configuration or anti-spam measures.
Unverifiable addresses can often be caused by:
So Why Do You Get These Results and What Do They Mean?
Catch-all addresses are created to ensure that email addresses that are incorrectly typed can still be received. This means that both of these email address will still be received "firstname.lastname@example.org" and "waezsrdvgnjmlWezxtcybnjomplwz5rextcy7vi5exrct@somedomain.com", it is very unlikely that someone has this as their email address.
However, many email hosts no longer permit catch-alls since spammers often try to guess email addresses in the domain; when they guess incorrectly, the spam is delivered to the catch-all address. The volume of incoming messages may well exceed the receiving limits, resulting in the account becoming locked or in legitimate messages being deferred, delayed, or bounced.
Greylisting is a method of defending e-mail users against spam. A mail transfer agent (MTA) using greylisting will "temporarily reject" any email from a sender it does not recognise. If the mail is legitimate the originating server will try again after a delay, and if sufficient time has elapsed the email will be accepted. Whilst we endeavour to verify greylisted addresses, sometimes it is not possible whilst still offering timely verification results.
Verification operations on remote mail servers can sometimes fail for a number of reasons such as loss of network connection, remote servers timing out etc. Unfortunately, there is no way of knowing when the issue will be resolved but these conditions are usually temporary. Retrying verification at a later time will usually result in a positive response from mail servers.