How to configure server to add new Mail Relay

1) Take the backup of sendmail.cf & sendmail.mc

2) Modify the entry in sendmail.mc file

    SMART HOST ‘proxyserver FQDN’

Also add the following entry under masquerade section.    

dnl #      MASQUERADE_AS(`example.com')dnl   

vi /etc/mail/sendmail.mc

SMART HOST ‘smtp.example.com’

MASQUERADE_AS(`example.com')dnl

3. Update the sendmail.cf file using the following command

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

4. Verify the sendmail.cf file using grep if the mail relay server is added in the text file

grep DS /etc/mail/sendmail.cf

Note: It should look something like this

DS

5. Bounce the sendmail service using the following command.

/etc/init.d/sendmail restart