TiL. I always assumed emails were case sensitive, and doubly so if used as a username. I find it strange that you even discovered this 'wrong' behaviour on the site in question: you purposefully typed your email address with different casing when logging in vs. registering?
Although the standard requires the local-part to be case-sensitive,[1] it also urges that receiving hosts deliver messages in a case-independent manner,[2] e.g., that the mail system in the domain example.com treat John.Smith as equivalent to john.smith; some mail systems even treat them as equivalent to johnsmith.[3]
You'll find the footnote links at the Wikipedia article, I'm not going to paste the here.
So yes, if my email address is my username, then I would expect it to work the same in uppercase or lowercase. If my username is "like an email address, but not an email address" then you make the rules for your site.
