[avar]

The domain being in any case for the purpose of delivery is part of the standard, but there's nothing I know if which would prohibit you from implementing local delivery and routing in any way you want once the mail is accepted.

[written]

You have no guarantee that the domain part case is preserved from what the user wrote, if it's not defined to be case-sensitive. So you can do what you want, but the input data are not reliable.