Although RFC 3501 (IMAP protocol) treats folder names as case sensitive (with the exception of the "INBOX" folder) the actual behavior varies with different servers. Some servers allow case-sensitive folder names, while others treat names differing only in lower/upper case letters to be equal. It may depend on the platform or file system the server is running on. Moreover, it may even depend on the folder hierarchy where some parts are case sensitive (eg. personal folders), while others are not (eg. archive stored on a different file system). RFC 3501 provides no provisions for detecting whether a server or particular folder hierarchy is case sensitive or not. This all stems from the original design of the IMAP to work on a variety of existing message stores. It doesn't accurately reflect the current situation where many IMAP implementations are deployed as cloud services or where a message store format is dictated by a particular server implementation.

Similarly, clients don't always handle case sensitivity correctly. For clients that use the local file system as folder storage, the case sensitivity of the file system could play a role. Traditional MBOX or MailDir storage systems will behave as case-sensitive on most Windows systems thus causing issues when the server contains conflicting folder names differing in a letter case.

In eM Client we try to preserve the case of folder names as entered by the user or sent by the server. However, we do not allow two folders with the same name differing by the letter case to be present at the same level in a hierarchy as we have to work with a large variety of servers and interoperate with mailboxes accessed by multiple different clients (including mobile ones). In addition, we allow offline operation where we cannot immediately get feedback from a server on what operations succeeded and which ones didn't. This involves many use cases where interoperability problems could occur if full case sensitivity was allowed. The number of servers and clients properly supporting case-sensitivity folder names was deemed so small that we found supporting it was not worthwhile.