"some not"? If you are implying that Clinton's server held messages marked as classified when sent, could you provide some evidence to this effect? The Clinton campaign is quite adamant that this is not true.
And using your own insecure personal server for these emails essentially guarantees that it's been hacked by.. China, Russia, take your pick.
In fact there is a term called "born-classified" which applies to information about foreign governments provided by foreign sources. Here's some evidence:
http://www.reuters.com/article/us-usa-election-clinton-email...
For example, take the Wikileaks information dump, which came from a huge cache of State Dept. cables and has now been covered in hundreds of public newspaper articles. If one of her friends forwarded one of those articles to Clinton, boom, she now has classified information in her email. Why? Because those cables, despite being leaked, were never de-classified.
Another example: a lot of the stuff revealed by Snowden was classified at a very high level. It still is today, even though it is widely available public information. Can you see how this could cause problems for someone who is a government official, but has friends/family/contacts who wish to discuss these news articles?
The press is a well-known vector that leads to a lot of government staff having classified information in their unclassified email systems. Typically investigations are rare and prosecutions rarer, because it's basically impossible to prevent and does no practical damage (since the info is already out there).
This is why the mere fact of having classified material on an email server does not necessarily imply incompetence or malice.
According to CNN:
> Two government agencies flagged emails on Clinton's server as containing classified information, the inspector general said, including some on "special access programs," which are a subset of the highest "Top Secret" level of classification, but are under subject to more stringent control rules than even other Top Secret information."
Ref: http://www.cnn.com/2016/01/19/politics/hillary-clinton-email...
As far as I can tell, the article to which you refer says nothing of the sort. It says that the email contains information which was classified after the fact. The article is based on the Inspector General's letter (http://i2.cdn.turner.com/cnn/2016/images/01/20/2016.01.14.-....), which says the same thing. Do you have evidence that the information was classified at the time it was mailed to her server? Her campaign says it was not.
This matters because a lot of publicly available information--stuff we all know, stuff you can read in the paper today, or can be easily found on Wikipedia--is nevertheless considered classified information. There is no possible way that a single person can keep it all straight in their head, which is why State staff (and others) rely on the markings on documents (digital or otherwise) to know how to treat them.
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FROM: Important Official TO: Hillary Clinton
MESSAGE: Info you requested - JFK is still alive. He wants to meet you soon. Also, aliens. Lots of them.
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In that ridiculous example, it seems like the Sec of State isn't thinking to herself, "I hope they don't classify this info later."
It's the official's responsibility to know what is and what isn't classified whether it is "marked". As a lawyer (and a Clinton!), she would certainly be parsing her words carefully.