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Category 9 includes, among other cases, Out-of-Band (OOB) bounces that cannot be safely machine-classified as hard bounces.
An OOB bounce message is generated after successful initial delivery of a message to the recipient's MX server (that's the server publicly advertised as processing mail for the domain). The MX may have signaled thumbs-up, but a later "hop" in the SMTP relay chain (which can involve 3-4 hops) wants to revise that to a thumbs-down. It can't alter the original response -- that connection has long been closed -- so it generates a new, outbound message, embedding in the message body both technical and machine-readable reasons for the rejection.
In fact, only an OOB bounce can properly be called a "bounce." Other forms of message rejection do not involve a separate inbound error message, so there is no "bounce message" in those cases.
Unfortunately, while an inbound bounce message can contain info that's useful to a human, a machine may not be able to parse it without potential errors. Knowing only that something came back after delivery, it's safest to consider this a soft bounce. Although hand-inspection would probably tell you that repeat OOBs are bad addresses, we don't have that ability.
A service like QuickEmailVerification can't know that the initial thumbs-up would later be revised to a thumbs-down, because it doesn't wait for a response message. In fact, it doesn't send a message at all, so it can't even know if the MX would accept a message, only that the email address appears to be emailable.