It may seem that a client implementation is vastly easier, but that is an illusion. Some of the complexity arises from the way IMAP imposes no specific naming

It may seem that a client implementation is vastly easier, but that is an illusion. Some of the complexity arises from the way IMAP imposes no specific naming model for mailboxes, so the client has to be prepared for that. But most of the complexity comes from dealing with offline operation and attendant state synchronization in a way that isn't horrendously inefficient.