[otrs] Agent notifications are not sent

Hi there, one of my otrs systems is not sending its agent notifications. After receiving a message there is the following log entry Sent agent 'NewTicket' notification to 'xxxxx' xxxx ist the correct maildress. But i dont get the notification Mail. Sending an "AdminEmail" works fine. Sending answeres and forwarding messages works fine. Any ideas to this riddle ? Matthias Wegner