I've tried to open the same data using the latest build of TRUNK, and it still gives an OutOfMemory error, using a limit of 10GiB of heap space (the test machine I have access to doesn't have 16GiB of RAM so I can't do an exact like-for-like test). Can this be correct behaviour when there shouldn't be more than 4.1GiB of messages in RAM at any point?
I'm not sure without seeing a sample of the data.
Any way you could share your data with us? (We never look at the actuall body messages.. they are all just random bytes for us).
(however I wouldn't like to get any data that will create liability on me even though I wouldn't look at the data, say if you are sending passwords or other financial sensitive data. Maybe you could the export tool to mask the data if that's the case (after some editing)).
BTW: The ExportTool may come handy to you on analyze the data.