A state department inquiry has accused Hillary Clinton and other former US secretaries of state of poorly managing email security.
Mrs Clinton failed to comply with rules on record-keeping, the inspector general found, and used private email for official business without approval.
But there are "longstanding, systemic weaknesses" in record-keeping that pre-dated her tenure, it said.
Mrs Clinton said the report showed she was no different from her predecessors.
"Contrary to the false theories advanced for some time now, the report notes that her use of personal email was known to officials within the department during her tenure," said a statement from the campaign.
It added that there was no evidence of any successful breach of her server.
The FBI is also investigating whether there was a security breach but has not yet interviewed her.
She has denied handling any classified information in her private emails.
The likely Democratic presidential nominee's use of private email has been a talking point in the election, with critics saying Mrs Clinton believes she is above the law.
Mrs Clinton's campaign says it showed that her practices were consistent with those of other secretaries of state who "also used personal email" and she was "not unique" in doing it.
State Department spokesman Mark Toner said that the report makes clear that the agency could have done a better job preserving emails and records, going back several administrations.