It's the job of Rob Russo, 30, who's been a close aide to the former secretary of state for the past decade, to respond to these letters.
He's written over 110,000 letters for Clinton since 2008, but the post-2016 election letters have been very different and difficult at times.
Russo told BuzzFeed it's been an emotional task, as the letters took on a much more personal tone following Clinton's election defeat.
I've been so blown away by the eloquence of the American people in the aftermath of this campaign. The quality of the writing...I mean, we still get casual 'I'm a fan!' or 'I love you!' Short things. But we get so many that are beautifully written — like people really sat down and took time to write something.
Russo is clearly very dedicated to Clinton, and it seems she's relied heavily on his assistance for years.
After the election, when Clinton began forwarding emails for Russo to print or save for a response, Russo told her he was "here to help," and she reportedly replied, "You always are. And that means so much to me."
Russo apparently has a relative amount of freedom with how he responds to Clinton's letters, but she is "is insistent that every message should get a response."
Since BuzzFeed's story came out and the person behind Clinton's letters has been revealed, people have taken to Twitter to express their gratitude.
The letters Clinton now receives reportedly feature an array of emotions and themes.
Some are angry at Donald Trump, others ask where she is, some simply tell her they miss her.
One person even sent a turkey on Thanksgiving, which was a nice thought but still had to be thrown away.
Whatever the message, it's Russo's job to ensure the sender gets a response.
"It's just what I do every day," he said.