Transferring mailboxes from hard drive to Macbook any suggestions?

I have a Macbook pro early 2011 running Lion. The hard drive was failing so it has been replaced and the original one put into an external drive for purposes of saving as much data as possible. However, since receiving back the Macbook and hard drive the graphics chip has caused a problem and so it has been sent off for 'reballing'? This is all way beyond my comprehension however although I cannot afford to have the work done through Apple I do trust the guy doing the repairs and am fairly confident that I'll end up with something usable. In the meantime I am using a white macbook also on Lion but I am really missing all the mailboxes on the Macbook Pro which contain such things as old emails with settings and account details. Are these stored in a movable file or is there any way I can open Mail using this external drive. I have no other backups. Hopefully this makes sense.