First I reproduced this bug on 2.3.0rc3 and 2.3.0rc4 and it's easy to reproduce it every time.
To reproduce it:
1/ Start James and create a user "userTest".
2/ Send 5000 messages to "userTest"
3/ Do a POP TOP to retrieve all the headers of messages from "userTest". To do this, use a software as Outlook. Configure it to retrieve mail from "userTest" and then click to retrieve the headers ONLY. DO NOT retrieve all the content of the message, otherwise you will do a POP RETR which is not a POP TOP! (Nota: the POP RETR is actually working, and I know that most of the time, people do retrieve all the content of the mail and not only the headers.. But it should work)
Finally you just have to wait the crash of JAMES. It will appear around the 3500 message...
The error is written in the log folder in the file connections. The error is OutOfMemoryError: JAVA HEAP SPACE