  Reply suggestion systems represent a staple component of many instant
messaging and email systems. However, the requirement to produce sets of
replies, rather than individual replies, makes the task poorly suited for
out-of-the-box retrieval architectures, which only consider individual
message-reply similarity. As a result, these system often rely on additional
post-processing modules to diversify the outputs. However, these approaches are
ultimately bottlenecked by the performance of the initial retriever, which in
practice struggles to present a sufficiently diverse range of options to the
downstream diversification module, leading to the suggestions being less
relevant to the user. In this paper, we consider a novel approach that
radically simplifies this pipeline through an autoregressive text-to-text
retrieval model, that learns the smart reply task end-to-end from a dataset of
(message, reply set) pairs obtained via bootstrapping. Empirical results show
this method consistently outperforms a range of state-of-the-art baselines
across three datasets, corresponding to a 5.1%-17.9% improvement in relevance,
and a 0.5%-63.1% improvement in diversity compared to the best baseline
approach. We make our code publicly available.
