Where domain names case sensitive then?
Names such as "Bell-ATL.com" are listed which use capitals as they please. Is this just so we can recognize the company name more easily or are they actually registered like that?
And #45, "Data IO.com", seems to have a space in the middle.
No, domain names have always been case-insensitive, just like e-mail addresses.
E-mail addresses are case sensitive.
The domain-name part of an email address is case-insensitive.
Theoretically, this could be true. Are there any email systems that are actually case-sensitive?
Yes, a lot of older systems (non-Unix) were case sensitive.
I'm going to guess Notes was one of them, but do you have any specific examples? (Unix™-derived e-mail systems being case-insensitive (although, I'd wager that /bin/mail was case-sensitive back in 7th Edition) is obvious, and I just checked our Exchange here on campus and it, too, cares not about the case of my recipient address.)
RFC 822 calls for the "local-part" to be case-preserved. It also specifies that the special address, "Postmaster", must be deliverable with any combination of case used. I do see, however, that RFC 5321 not only reinforces the idea that the case of the "local-part" must be preserved, but it explicitly declares that "smith" and "Smith" may very well be different recipient mailboxes, which is brain-damaged, given that those are very often typed in by fallible humans.