You probably changed the hostname of your machine, here is how to make it work with your new hostname...

Open up /etc/sendmail.cf

look for the following code...

###################
#   local info    #
###################

Owlocalhost

Everything after Ow should be the hostname of your machine.

Next look for...

# my official domain name
# ... define this only if sendmail cannot automatically determine your
domain
Dj$w.something.here

Everything after Dj$w. should be your domain (use 'localdomain' if you
don't have one)

Close sendmail.cf.

Now open up /etc/sendmail.cw and make sure your hostname and domain name
are listed in there together (hostname.domainname).

Close sendmail.cw.

Lastly open up /etc/hosts

Make sure this file also has your hostname and domain name entered like
so:

127.0.0.1       yourhostname     yourhostname.localdomain
127.0.0.1       localhost  localhost.localdomain