Moved: This thread is more suitable in Linux-General and has been moved accordingly to help your thread/question get the exposure it deserves.
To mount your old Linux partition you need to know which one is it first. Do it using fdisk, this way
fdisk -l /dev/hdb
/dev/hdb means primary slave disk. If it's a different one, you need to change it in the command. The result will be a table with device name, size and filesystem (Linux partitions are shown as 'Linux'). Find your Mandrake partition on the list. Size may probably help. When you have it run
(to create a new directory)
mount /dev/hdbX /mnt/mandrake
Replace X with the right value from above.
Which mail program have you used? Their mail directory locations are different.