When you login into your ubuntu server and a message appears that you have mail than you’ve got a system mail. To read system mail you need to install a command based mail client. I suggest you to install a tiny mail client called mail which is part of mailutils.
Install mailutils by typing:
sudo apt-get install mailutils
Now run mail to read your system mails:
Do delete a mail just press d and the mail number like this:
d 1