Sure they are. They were inside Hillary Clinton’s house, which itself was guarded by the Secret Service. Furthermore, we have it on the highest authority that there was nothing untoward on those servers. Clinton has said, after all,

I did not email any classified material to anyone on my email. There is no classified material. I’m certainly well-aware of the classification requirements and did not send classified material.

Ex-UN Ambassador John Bolton had some thoughts on one thread of emails, and he disagreed with the UNCLASSIFIED marking at the bottom of the first email in the thread.

I think the information in the email is clearly classified. If I were engaged in the negotiation on that subject reporting back to Washington, my reporting cable would be classified.

They’re dealing with the possible US military operation, sensitive negotiations among NATO partners, talking about US objectives and political arrangements and possible objections to the deal from key partners so all of these at secretary of state level is extraordinarily sensitive.

Apart from hackers, the security of an email server is only as good as the security of the users of the email server. When folks send classified email, the server(s) handling it become classified, whether that was the intent or not.

Clinton is mistaken in claiming her personal server did not handle classified emails.