When a client attempts to log in with an incorrect client version for the server, the following message is written to the server log/console:
Connection closed: Error during SSL handshake: error:1408A10B:SSL routines:SSL3_GET_CLIENT_HELLO:wrong version number
This is pretty definitive for helping the server operator for determining the issue. However, the client user does not receive any message mentioning that they may have the wrong version, all they receive is a disconnected message.
This is an opportunity to allow client users to self-help. I have no idea why so many users try to connect with an old version; I have no idea where they are getting it. This seems to happen often, and I get called in to resolve an issue that the client user could fix themselves with the smallest bit of direction.