Disable local delivery Linux

What I did to disable local delivery. I'll be using the example.com domain.

Requirements:

• example.com A entry pointing to IP address assigned to one of the eth interfaces.
• /etc/hosts defining example.com assigned to the very same IP address as above
• example.com MX records pointing to Google servers (ASPMX.L.GOOGLE.COM, etc)
• default sendmail installation (mine was on Ubuntu)

Steps:

vim /etc/mail/sendmail.mc

at the end:

define(`MAIL_HUB', `example.com.')dnl
define(`LOCAL_RELAY', `example.com.')dnl

and then:

sendmailconfig (or /etc/mail/make depending on your distro)
service sendmail restart

testing:

echo -e "To: user@example.com\nSubject: Test\nTest\n" | sendmail -bm -t -v
echo -e "To: user\nSubject: Test\nTest\n" | sendmail -bm -t -v

You should see it connecting to the google server and then you should see your mail being delivered to your Google inbox.

Copied from : http://serverfault.com/questions/65365/disable-local-delivery-in-sendmail