For security reason sendmail is by default configured to accept connection from local system (127.0.0.1). This should avoid open mail relay problem.

To allow connections from ALL hosts/LAN IPs open sendmail.mc file (login as the root):

# vi /etc/mail/sendmail.mc

Look for line that read as follows:

DAEMON_OPTIONS(`Port=smtp,Addr=127.0.0.1, Name=MTA’)dnl

Comment or remove above line and insert new line that read as follows:

DAEMON_OPTIONS(`Port=smtp,Name=MTA’)dnl

This line avove will force to accept connection from any host. Save the file. Regenerate sendmail configuration file using m4:

# m4 /etc/mail/sendmail.mc > /etc/mail/sendmail.cf

Finally, restart the sendmail service :

# /etc/init.d/sendmail restart