The current version of Ethereum addresses are case-insensitive.
They also don’t have any characters dedicated to a checksum, to make sure you typed the rest correctly. The case-sensitive address is an added protocol that allows a normal address to be represented with some data that allows software to recognize if it’s a valid address or not.
That just means that the mixed-case addresses you see are more secure, but they still both work, it’s just a little scarier to send to an all-lower-case address.
For example, address 0x6eaff3479c17b8e7a0069823dc87bab09fbf4549 is valid, but 0x6eAff3479c17b8e7A0069823DC87bAB09fBf4549 is easier to copy down safely.
You can read more about this format in EIP 55.