I immediately assumed "well, gee; the RFC (2822) should answer this definitively". So I looked and found out that "The local-part portion is a domain dependent string. In addresses, it is simply interpreted on the particular host as a name of a particular mailbox." There is no definition of how that interpretation works. I also thought emails were always lower case, but then realized that _my_ ISP says it translates to all lower case, but there is no such requirement in the RFC.
I still think it is bad practice to blur something that people think of as a unique identifier.