All services use the same verify to the same primary status codes:
OK: Verification passes all checks including Syntax, DNS, MX, Mailbox, Deep Server Configuration and Grey Listing (bulk only).
Bad: Verification fails checks for definitive reasons (e.g. mailbox does not exist).
Unknown: Conclusive verification result cannot be achieved due to mail server configuration or anti-spam measures.
Catch-all: Usually refers to a mailbox on a domain that will "catch all" of the emails addressed to the domain that do not exist in the mail server. Configuring a catch-all address can help avoid losing emails due to misspelling. However many email hosts no longer permit catch-alls since many spammers abuse the feature, spamming random email addresses. Catch all configured servers cannot respond to requests for email address verification.