I understand ur problem, but tell me your Linux operating system Flavour name and tell me the version also.

But if u r using Redhat linux 7.X then

add a line in /etc/sendmail.cf
DSmail.x.x.net

and there is also add a option

O DaemonPortOptions=Port=smtp,Addr=192.168.10.1, Name=MTA

192.168.10.1 is ur lan ethernet address.

and the next thing is that,

in the /etc/mail/access

there is a file u add folowing entries
localhost.localdomain RELAY
localhost RELAY
127.0.0.1 RELAY
192.168.10 RELAY

192.168.10. is the network and may be u have differnet. i hope that ur problem is solved.