My Machine can't receive mail

["Reuben D Budiardja" <reubendb goshen edu>]

Hello,
It seems that my machine cannot receive mail from outside. I am in LAN 
network, and my machine has its name from DHCP, which stays the same. I tried 
to change the file /etc/mail/local-host-names and add my machine name there:

machine_name.domain.edu

and restart sendmail, but no result. When I send e-mail to 
reubendb machine_name domain edu, it will just gone. 

Any help on this? I am using Redhat 7.2, with the default sendmail included.

Thanks in advance.
Reuben D. Budiardja