"Pls use this for reply," Clinton wrote in her email, sent from the clintonemail.com account she set up on an unsecured, private server in her New York home for her work as secretary of state.

Over the following hours, Mitchell wrote back to Clinton with summaries of his conversations, including one with Spain`s foreign minister, who had briefed him on discussions with Palestinian leaders. The State Department has redacted the summary of the minister`s thoughts, saying it is classified information.

The exchange is among dozens in a new batch of Clinton`s emails released this week that shed further light on how Clinton handled information while she was secretary of state from 2009 to 2013.

Clinton, front-runner to be the Democratic candidate in the November 2016 presidential election, has faced steady criticism from political opponents since it emerged in March that she used a private set-up rather than a government-issued state.gov email address.

Clinton has maintained she did nothing wrong. She says she sent no information via email that was classified at the time, and received no material marked that way.

The Federal Bureau of Investigation is examining Clinton`s server to see whether information was mishandled. No evidence has emerged suggesting Clinton`s email practices harmed national security.

A review last month by Reuters of previously released Clinton emails found 30 email threads that the State Department has marked to show they include information shared in confidence by foreign government officials, from prime ministers to spy chiefs.

U.S. government regulations examined by Reuters say this sort of information, whether written or spoken, must be classified from the start, and handled through secure, government-controlled channels.

The Clinton-Mitchell correspondence is one of 57 email threads found by Reuters in the latest batch of emails released on Monday that the State Department has marked as including the same type of information.

In all the 87 email threads examined by Reuters, the State Department has blanked out the confidential information in the public copies, adding the classification code "1.4(B)", denoting foreign government information.

This is the only kind of information that presidential executive orders say is "presumed" to likely harm national security if wrongly disclosed. State Department regulations describe it as the "most important category of national security information" its officials encounter.

If the State Department`s markings are correct, it appears that Clinton and her senior staff routinely did not follow the regulations in the department`s Foreign Affairs Manual, which tells employees they "must" safeguard foreign government information by treating it as classified.

"It`s hard to square the secretary`s conduct with the strict letter of the FAM," Steven Aftergood, the director of the Federation of American Scientists` government secrecy project, said in an email.

The department and spokesmen for Clinton have declined requests to explain this apparent lapse.

UNCLEAR HOW CLOSELY REGULATIONS FOLLOWED

It is not clear if Clinton approached classified information differently than other secretaries of state before or after.

Several career diplomats, who joined the department before Clinton`s tenure, also sent foreign government information through their unclassified .gov email accounts, the marked redactions on Clinton`s emails show, suggesting that the regulations may be commonly ignored in favor of speedier communications.

Asked whether John Kerry, the current secretary of state, has sent such information via unsecured channels, a State Department spokesman declined to say either way.

The department declined to say whether Clinton adhered to the relevant regulations and laws while she was in charge, or whether the secretary of state is even bound by the department`s regulations.

The department has said the information in some of Clinton`s emails is being newly classified now, but it has also said it cannot know for sure whether the information should have been handled as classified all along.

Nearly 8,000 emails have been released, about a quarter of the total Clinton returned to the department last year. They are being published in monthly batches following a federal judge`s orders. The State Department has redacted classified information from nearly 200 of them so far. All but one of them is marked "Confidential," the lowest level of classified information.

Several emails show Clinton and her staff were mindful of handling sensitive information with care: there are repeated references to setting up conversations over secure telephone lines.

In a September 2010 email, Jake Sullivan, a senior aide now working as an adviser for Clinton`s presidential campaign, tells Clinton he just met with Israeli Prime Minister Benjamin Netanyahu for several hours.

"Happy to talk secure at your convenience," he wrote. He went on to summarize "highlights" from the conversation. Five or six lines of text follow, all now blanked out, classified and stamped "CONFIDENTIAL."

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