I had been working on a tool to read the messages in a gmail account and create a spreadsheet with all the bad to/from/cc/etc headers for the user to update. However I just recently tried a pst->gmail conversion tool someone has written http://code.google.com/p/pst2gmail . Having previously copied the emails from the PST to gmail via Outlook and experiencing the "unknown sender" problem, I didn't think to try it before. Surprisingly it started importing the emails with the headers intact! It spat out errors quite frequently so it is not entirely bug free, but I will probably try to contribute to that project going forward. Since it may work with no problems for other folks, I thought I'd mention it here.