I have noticed that auto unrar is failing to unrar some multi-part messages.
I maintain a separate download folder for the incoming files, and noticed that one the multi-part file has been downloaded, the .rar files are immediately moved to a sub folder called Loaded, but not the .par files, which then causes the other .par files to be download to "repair" the download. However, the .rar files are fully intact and can be manually unrarred from the "Loaded" subdirectory.
Not sure if this is making sense, but I can provide more details if needed.
Thanks for the great product!