Emails in this category are of the best quality. It has been verified that messages sent to these addresses will be accepted by the responsible mail server and it has also been verified that the mail server will refuse invalid addresses.
Disposable emails are valid emails, but from the mailing list owner perspective, they are mostly useless. These emails are hosted by servers such as Mailinator, 10 Minute Mail, TrashMail, etc. Users of disposable email services use their emails addresses for very short period of time, commonly only for the purpose of a single registration. It is very common that messages delivered to such addresses are never read by humans and are deleted within couple of minutes.
Emails in this category are accepted by the responsible mail server as valid emails, but the mail server accepts emails to any email address on the given domain. It is thus impossible to distinguish between valid and invalid emails on that domain. If email addresses in this category were obtained using a reliable list building method, such as the Double Opt-in Process, they may be treated as valid emails. Otherwise, it is better to avoid using them.
Emails in this category have a good syntax, but they are reported as invalid or unusable by the responsible mail server and messages to them won't be accepted. There are many reasons why a previously valid email address can end up in this category. For example, the email account could be cancelled, but it could also happen that the email account still exists but nobody is retrieving emails from that address and it is now full and new messages can't be delivered to this address and will bounce.
Items in this category have not passed the syntax check and thus can't be treated as email addresses. For example, an email address can not contain space characters, and thus "john firstname.lastname@example.org" is not a well-formatted email address.
It was not possible to verify emails in this category. There are several reasons how this can happen. The most common situation is that the responsible mail server is configured in a way to refuse cooperation with email validating services. The mail server cooperation is required for the verification process to complete successfully. Another reason can be that the resposible mail server is not currently reachable, e.g. it is down due to maintanence or due to network error. Treat these emails as emails hosted on Catch all domains – if they were obtained using a reliable method, it might be OK to use them, but avoid them otherwise.