The first major theme of the novella is the concept of communication. The main character's job is relay messages between senders and recipients. However, the protagonist inserts herself into the communication loop by trying to figure out the lives of the customers through their messages. In a sense, she is an eavesdropper but views her fascination with their lives in the way an anthropologist would study a tribe or culture through observations from afar.

The other major theme of the story is the dichotomy between her personality and the lives of her customers. The main character is an introvert, whereas her clients are part of the cosmopolitan London society. She is fascinated by their lives because they are different from hers. A vivid metaphor for this separation of lives—hers and the customers'—is the "cage" that she works in, which is a small room in the post office dedicated solely for telegraphy. She is physically and socially separated from her customers, and yet she is in close proximity to their personal lives.

Another prominent theme of the story are the secrets the protagonist relays and keeps for her clients. The messages themselves are cryptic, and she is able to decipher the codes to gain insights into their lives. When she promises a particular client that his secret messages are safe, she inadvertently inserts herself into their personal lives. The codes in the messages are part of the linguistic and communication philosophies that Henry James attempt to explore. Communication—semantics, lexicon, and semiotics—is not just a way for people to express their thoughts and relay commands, but is also a way for people to hide those expressions from third parties.

Finally, there is the theme of relationships. The protagonist is, at first, reluctant to marry her fiancé. However, after learning about the complex marriages and affairs of her clients, she decides to marry him in the end. The protagonist was able to learn about marriage dynamics through her observations of other people.