As Dave_L says it would be good to see a portion of the csv file.
There will also be some problems with preserving the threads. It seems that there is no way to see which thread a post belongs to except by the subject in the first format. If there are threads with the same subject they will be merged into one. All replies in a thread will also be a reply to the first post of the thread after the import, but hopefully that won't be much of a problem.
In the second format there won't be a problem separating the threads since there is one thread per mail. On the other hand there might be some problems parsing since there is no identifying text in front of the subject or where message ends. But if no message has "Re(1):" (or any number, if this changes in every message) at the start of a line in them this shouldn't be a problem either.
