Clinton has said it was a mistake for his wife Hillary to maintain a personal email server even though her predecessors and her successor at the US State Department did the same.
The former US president said Mrs Clinton should have known that there would be a different set of rules applied to her if she ran for the White House.
Mr Clinton defended the Democratic presidential nominee during a question-and-answer session at a Las Vegas forum organised by the Asian American Journalists Association and APIA Vote.
He said it did not occur to diplomats sending the emails at the time that they should be concerned with records classification.
Mr Clinton also said the email server should not be a cause for distrust and that people in the national security community would not endorse his wife’s candidacy if it was.
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Meanwhile Republican rival Donald Trump said the only way he could lose in Pennsylvania is if there was cheating.
The billionaire property tyvoon told a rally in Altoona that he planned to watch certain parts of Pennsylvania closely on election day in November.
He noted that he has had huge turnout for rallies in the state as evidence of what he says is overwhelming support.
But Mr Trump said he wanted to ensure that Mrs Clinton’s supporters did not vote multiple times, citing concerns about voter fraud.
He also claimed the National Security Agency had Mrs Clinton’s missing emails and suggested that the agency release them publicly.
The NSA has never said that it has Mrs Clinton’s emails and Mr Trump appears to be referring generally to the widespread assumption that it has information about millions of people.
Mrs Clinton’s legal team turned over more than 30,000 emails from her server to the State Department last March, but only after deleting another 30,000 messages that the legal team deemed private and personal. The FBI has reconstructed some of the missing emails.