Collaboartion Services uses the mailbnickname value from the orginal source object to genereate the legacyexchangeDN value for the stub object on the subscriber side.
If the mailnickname attribute contains unusual characters such as characters like the german umlaut this will be present in the legacyexchangeDN value for the stub objcet.
Work around: change the mailnuckname value to remove the german unlaut characters and then republish the collection. This will remove the umlaut character from the legacyexchangeDN and you should then be able to send email to the user.
