A system and method for clustering nodes of a tree structure is provided. A plurality of messages is maintained. Each message is represented as a node in a tree structure. A word vector is assigned to each message. Pairs of the nodes are identified based on relationships in the tree structure. The nodes of one or more of the pairs are combined into clusters. Boundaries of each cluster are adjusted, including at least one of placing a root node into one such duster having a closest related child node, separating children nodes into distinct groups and retaining a relationship between a parent node and one such group including a nearest child node, and transferring a parent node to one such cluster having all children of the parent node. A digest of the messages, including one or more of the clusters is formed.