How to import my old mail into Gnus?
The easiest way is to tell your old mail program to export the messages in mbox format. Most Unix mailers are able to do this, if you come from the MS Windows world, you may find tools at http://mbx2mbox.sourceforge.net/.
Now you've got to import this mbox file into Gnus. To do this, create a nndoc group based on the mbox file by saying ‘G f /path/file.mbox RET’ in Group buffer. You now have read-only access to your mail. If you want to import the messages to your normal Gnus mail groups hierarchy, enter the nndoc group you've just created by saying ‘C-u RET’ (thus making sure all messages are retrieved), mark all messages by saying ‘M P b’ and either copy them to the desired group by saying ‘B c name.of.group RET’ or send them through nnmail-split-methods (respool them) by saying ‘B r’.