I'm using $domain as the key for a lookup in a rewriting rule, but its contents are not being lowercased. Aren't domains supposed to be handled caselessly?

The value of $domain is the actual domain that appears in the address. It could of course be lower cased, but I know that would cause some unhappiness, because some people have mixed-case domain names which look silly if the case is changed. Thus, one wants to preserve the case in rewrites such as

*@*.TheRap.com   something@$domain

because therap doesn't look like two words. I know it seems trivial, but it is important to some people - especially if by some unfortunate accident the lowercased word is something indecent. You can trivally force lower casing by means of the ${lc: operator. Instead of $domain write ${lc:$domain}.