Smart Reply uses machine learning to figure out if a short Reply is appropriate and then predicts likely responses. "The Smart Reply System is built on a pair of recurrent neural networks, one used to encode the incoming email and one to predict possible responses. The encoding network consumes the words of the incoming email one at a time, and produces a vector (a list of numbers). The second network starts from this thought vector and synthesizes a grammatically correct Reply one word at a time, like it's typing it out. Amazingly, the detailed operation of each network is entirely learned, just by training the model to predict likely responses."