“Given all the power she had as secretary of state, a lot of that work would be jumbled together,” Mr. Wonderlich said. “Her presidential ambitions and the family foundation would be wrapped up technically in email.”

Mrs. Clinton’s allies have maintained that she followed protocol in the use of a private email address. A spokesman declined to elaborate on Wednesday about her use of clintonemail.com for matters related to the Clinton Foundation, which has received millions of dollars in donations from foreign governments. The foundation ceased to accept most donations from foreign countries while Mrs. Clinton was at the State Department but began the practice again after she left office in February 2013.

In an email of talking points to supporters, Burns Strider, a senior adviser to Correct the Record, a group that defends Mrs. Clinton in the news media, pointed out that former Gov. Jeb Bush of Florida, also a likely 2016 presidential candidate, also hosts his own personal email server.

Mr. Bush is a prolific user of email who continued to use his personal jeb.org domain, which his aides could also access, while he was in the governor’s office, said Kristy Campbell, Mr. Bush’s spokeswoman. Under Florida’s records laws, emails from Mr. Bush’s personal account have been made public. “His emails were available via public records requests throughout his time in office and have remained available,” Ms. Campbell said.

In earlier years, Mrs. Clinton’s account at clintonemail.com was connected to a server registered to the Clintons’ Chappaqua home in the name of Eric P. Hothem. Mr. Hothem, a former aide to the Clintons, now works in finance in Washington, according to regulatory disclosure documents.

Mr. Hothem, whose name was misspelled in Internet records, did not return a message left on Wednesday with an assistant at his office. Mr. Cooper, whose name is on the clintonemail.com domain registration, now works at Teneo Holdings, a corporate advisory firm with a broad array of global business clients partly run by Douglas J. Band, a former adviser to Bill Clinton.

The Clintons eventually decided they did not want all three family members on the same email domain, in part, an adviser said, out of concern that it might look as if Mrs. Clinton’s official business at the State Department was too closely overlapping with Mr. Clinton’s work as a global philanthropist. Mr. Clinton stuck with presidentclinton.com, which was established in 2002. Chelsea Clinton has now set up chelseaoffice.com. The clintonemail.com domain is set to expire in 2017, when Mrs. Clinton, if successful in her presumptive campaign for president, would take office.