Where does the mail utility store read mail?
If you wanted a full featured editor / mail client from the command line:

brew install mutt


If you just want to see the mail you can use the from command to see headers or look at the files in /var/mail/${USER}. Depending on how you disposed of messages, they also might be in your ~/mbox file

Some good reading at:


OS X: Read mbox formatted files in /var/mail (local mail) graphically
Where is my mbox located at?
https://superuser.com/questions/306163/what-is-the-you-have-new-mail-message-in-linux-unix