Every email is primarily categorized as either Valid, Invalid, Unknown or Catch-All.
Valid - After a successful SMTP transaction, if receiving mail server accepts the email address, it will be marked Valid Email Address. Sending emails to the addresses which are marked as "Disposable" is not recommended, even though the email address is valid.
Invalid - An email address will be marked Invalid if it is syntactically incorrect or an email account does not exist on receiving mail server.
Unknown - Sometimes receiving mail server responds very slowly or may temporarily be unable to process requests.
Catch-All - A catch-all email address is an account that receives emails sent to any incorrect email addresses of the domain. When configured, emails sent to any email address associated with the domain should not bounce, but in some cases, it may reject later.