Which major dictionary publisher pioneered the 'Word of the Day' email subscription as a loyalty program that later became an industry standard practice?

In the world of linguistics and language publishing, loyalty programs have been used to maintain customer bases and encourage continuous engagement with reference materials. In an age where digital resources are often free, traditional dictionary publishers have had to innovate their customer retention strategies. This question explores a notable loyalty initiative that changed how we think about dictionary subscriptions.

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Merriam-Webster
Oxford University Press
Cambridge University Press
Collins Dictionary