Mrs Clinton has released her tax return and criticised Donald Trump for failing to do the same
Bill Clinton has said it was a mistake for Hillary Clinton to use a personal email server while secretary of state even though her predecessors and her successor did the same.
The former president said his wife should have known she would be subjected to different rules if she ever ran for the White House.
Mr Clinton defended the Democratic presidential candidate during a question-and-answer in Las Vegas, arguing it did not occur to diplomats sending emails at the time that they should be concerned with records classification.
He also insisted the private email server controversy was not "a cause for distrust".
"If it were a cause for distrust, it's inconceivable that all these prominent national security people ... would've endorsed her," he added.
Mrs Clinton's email system, which she used during her four years in office, has continued to be a source of controversy during the presidential race.
She maintains it was set up for convenience, but critics claim it was because it gave her total control over her correspondence and what information should be released, such as to the congressional committee investigating the 2012 attack on the US consulate in Benghazi.
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 were deemed private and personal.
Meanwhile, Mrs Clinton has released her tax return and criticised her Republican rival Donald Trump for failing to do the same.
The document showed she and her husband reported an income of $10.6m (€9.5m) in 2015.
Clinton campaign communications director Jennifer Palmieri said: "Donald Trump is hiding behind fake excuses and backtracking on his previous promises to release his tax returns.
"What is he trying to hide?"
But Trump senior communications adviser Jason Miller said people would rather see the deleted emails from Mrs Clinton's private server.
He said: "We want to see the records the night of Benghazi that explain why secretary Clinton didn't send in reinforcements as soon as the attack had begun."
Mr Trump, meanwhile, stepped up his warnings that the election could be rigged, claiming at an event in Pennsylvania that the only way he could lost the state "is if cheating goes on".
In a speech, quoted by The Washington Post, the Republican candidate said: "We’re going to watch Pennsylvania, go down to certain areas and watch and study, and make sure other people don’t come in and vote five times."