Enter Ofaco. The name “Ofaco” stands for “Old-fashioned completion”, which says it all: it’s a plugin for Mail that replaces Mail’s dictionary-based completion mechanism with one that uses the words from the message window from which it was invoked.

Additionally, I added a way to define default completions that will always be added to the completion list (and which, by the way, can not only contain words, but also phrases).

Ofaco is freeware. However, if you enjoy using Ofaco and would like to help support its development, please consider making a donation.

Requirements:
OS X 10.7+
Intel