For practical reaons all Postfix address rewriting table lookups are case-insensitive by default(*), both for the domain and local part, regardless of what the RFCs say.
(*) When creating an indexed table, the postmap command folds the
input to lowercase before writing to the table, and code that queries
such tables folds the key to lowercase. The pcre: and regexp: lookups
ignore case by default. Password file queries also use a case-folded
key.
Wietse
