[account42]

There are two parts where the case (in)sensitive distinction matters:

- what case you use to send mails

- what rules you use to determine if two email strings are the same user

For the first you can and should always use the address exactly as entered. For the second that's going to be a guess anyway and the exact rules depend on what false positives and false negatives mean for your use case - and you are going to have at least one of those two. Assuming case insensitivity here is generally reasonable for most use cases.