An “internet letter” is a colloquial term commonly used by the elderly when referring to an email. This term makes more sense to people who are unfamiliar with such new-fandangled technologies because the only thing an email really compares to: the traditional written letter. This term is not solely limited to use by the elderly; it can be used by anyone unfamiliar with email, such as those from the third-world, and those from extremely rural parts of the U.S.
My grandson just got me one of them personal computers with the internets already on there. Now, I can send internet letters to all my grandkids.