Nop LDAP is a protocol, it defines its attributes, mail is how ldap maps an email, there is an unique way of identify it, you can see a list of possible attributes here:http://www.computerperformance.co.uk/Logon/LDAP_attributes_active_directory.htm
Maybe it could be great to check that de authenticationAttribute is in that list, so the user can not make a mistake when configuring the plugin, but that is a plus