An argument against supporting all Exchange versions from 5.5 onwards, that I have heard, is that Exchange 5.5 is dead technology and that I might as well focus on Exchange 2000 onwards. This is far from being true. Exchange 5.5 are alive and well in many server rooms. Many organizations would really prefer to keep their Exchange 5.5 despite Microsoft discontinuing offical support.

The "good" thing about Exchange 5.5 is that it is known inside out by most Exchange admins. An acceptable option is therefore to isolate the old Exchange server behind trusted firewalls and just keep it running for as long as possible. Brutus enables an organization to use alternative desktop clients that supports a multitude of backends, thus migrating away from Exchange in small manageable steps instead of one giant "upgrade to Exchange X.Y" transformation.