There is a fairly simple problem with the doMailSize() method in SMTPHandler. When a MAIL FROM command is received with a SIZE value that exceeds the maxmessagesize value, the server sends a 552 response. It then attempts to log a message about the incident, and calls state.get(SENDER).toString() to form part of the log message.
Unfortuntely, state.put(SENDER, senderAddress) is only called later when the MAIL command is processed. Creating the log statement thus causes an NPE and the connection aborts.
The easy solution is to take this information out of the log message. I apologize for not including a patch, but I have lost track of which branch to patch against. However, I believe this problem is present in RC3.