When Mrs. Clinton’s aide, Huma Abedin, approached Mr. Clinton’s staff about piggybacking on the server, they were initially skeptical that it technically could handle the additional load. But requests from Mrs. Clinton were nothing they took lightly and they complied, people familiar with the matter said.

Mrs. Clinton took office in January 2009 and began using her private email system for both work and personal matters. Her office said the system used “robust protections” and “additional upgrades and techniques employed over time as they became available.”

Kevin Bocek, a vice president at the Internet security company Venafi, said the Clinton server was encrypting data it sent and received as of March 29, 2009, about two months after she took office, based on a search he did of Internet records. During the first two months of her tenure, however, it doesn’t appear that Mrs. Clinton’s email had such protections, Mr. Bocek said.