A plugin for Apple Mail that shows messages related to the currently viewed message. The RelatedMessages plugin builds an index, similar to Spotlight, of all your mail files, and uses that index to find messages related to the currently viewed message.

The plugin determines relevance by looking at both the headers and the contents of the message. A given item in the related messages list shows the sender, date, subject, and first sentence or two from the message.

Above the item is a score indicator – the longer the bar the more relevant the message. Various styles are applied to the text to indicate a particular relation to the currently viewed message.