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.
Wikipedia has a good summary on what is valid. https://en.wikipedia.org/wiki/Email_address
From the second paragraph:
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.