Most of the stolen emails were exchanges between her and Mr. Podesta, her former boss at the Center for American Progress, a liberal think tank that Ms. Tanden now leads. Mr. Podesta is a longtime friend, and Ms. Tanden’s tone is often that of a colleague grousing in the break room.

In the exchanges, Ms. Tanden can be fiercely protective of Mrs. Clinton. When Jennifer Palmieri, communications director of the Clinton campaign, wrote a sharply worded letter raising concerns about coverage of Mrs. Clinton in The New York Times, Ms. Tanden cheered. The letter “is great,” she wrote to Mr. Podesta — adding a sentence laced with profanities directed at the newspaper.

But she could also be direct about other members of Clintonworld and the candidate herself.

When news media reports began identifying potential members of Mrs. Clinton’s campaign team last year, Ms. Tanden warned of griping among Democrats about racial and gender balance among Mrs. Clinton’s staff picks. “I’m not the diversity police but there is grumbling on the 4 white boys running next presidential cycle,” she wrote in January 2015. “So I recommend rolling out some people who look like the rest of America soon!”

In May 2015, New York City’s mayor, Bill de Blasio, emailed Ms. Tanden and Mr. Podesta to let them know that he was scheduled to appear on the MSNBC show “Morning Joe” and would likely offer his opinion of Mrs. Clinton’s policy vision.

“I find him a bit insufferable,” Ms. Tanden wrote to Mr. Podesta. (The mayor was frozen out over the ensuing months and denied a prime speaking slot at the Democratic convention this summer.)