The mailx command without any options can be used to list the emails in your current users inbox.

~]# mailx
Heirloom mail version 12.5.7/5/10. Type ? for help.
"/home/john.doe/Maildir/": 2 new messages 2 unread
>U jane.doe              Mon Jan 01 09:00 17/565  "Hello John"
 U jack.doe              Mon Jan 01 10:00 17/532  "Hello dad"
&

The -u option can be used to list the emails in some other users inbox.

mailx -u jane.doe