Type "mail" in the terminal window and it will say whether you have mail or not. If no mail, it will automatically quit. Otherwise it will list the mail messages for the user you are logged-in as, each one uniquely numbered. Type "?" to see a list of commands. To read a message, just enter its number. To delete, type "d#" where # is the message number. If you "exit" mail, then nothing will be deleted; you have to "quit" instead. To mess around with mail for the first time, send yourself a message...
mail -s testing yourusername
Note the period as it indicates the end of the message. To check whether mail is functioning properly, read the log by entering "tail /private/var/log/mail.log". Each message will have at least two entries in the log, one ending with "relay=" and the other ending with "Sent" . Other entries are errors, some of which you can ignore (except it fills-up the log) as the mail gets sent anyway, but others can drive you crazy trying to figure-out what is wrong. It's important at this point not to blindly follow what you read about sendmail on the web because MacOS X (darwin) is not the same as Linux or other Unixes and your machine can get really hosed (I speak from experience :-)