MacAdam is a surname, like the Scottish engineer John Loudon McAdam who invented the road construction known as "macadam". "Sandy.MacAdam@example.com" comes across rather different than "sandy.macadam@example.com".
A hypothetical DrAbby@example.com probably would prefer keeping that capitalization over "drabby@example.com".
I'm sure there are real-world examples.
On a related note, I knew someone with an Irish O'Surname who was very particular that the computer systems support his name. (As https://stackoverflow.com/questions/8527180/can-there-be-an-... puts it, "People do have email addresses with apostrophes. I see them not infrequently, and have had to fix bugs submitted by angry Hibernians.") No doubt some of them also want to see the correct capitalization be used.
A possibly better alternative is to recommend that the normalization be used only for internal use, while using the user-specified address for actual email messages, and to at least note some of the well-known issues with normalizing to lower-case.
