I would suggest you have a look at the following site:-
http://www.sendmail.org
Lots of FAQ's etc on the use of Sendmail. For what you are after, it is dependent upon the version of Sendmail on your server. The following is contained in the 'relay' section of one of the FAQ's on that site.
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As of sendmail version 8.9, forwarding of SMTP messages is not permitted by default. For example, if you are on site A.COM, you will not accept mail from B.NET destined for C.ORG without special arrangements. These instructions assume that you are using the M4 technique for creating your sendmail.cf files; if you are not, you are on your own.
The simplest approach is to list the domains you are willing to relay in the file /etc/mail/relay-domains. Anything listed in this file will be accepted for relaying. N.B.: Sendmail must be restarted after this file is modified.
For more precise tuning, several FEATUREs have been added to control relaying:
FEATURE(relay_hosts_only). Normally domains are listed in /etc/mail/relay-domains; any hosts in those domains match. With this feature, each host in a domain must be listed.
FEATURE(relay_entire_domain). Setting this feature allows relaying of all hosts within your domain. For example, on the host gateway.A.COM, this feature allows mail to or from any host in the A.COM domain. More precisely, this relays any host listed in the $=m class. This is equivalent to listing the name of the domain in /etc/mail/relay-domains.
FEATURE(access_db). This enables the hash database /etc/mail/access to enable or disable access from individual domains (or hosts, if FEATURE(relay_hosts_only) is set). The database format is described below.
FEATURE(blacklist_recipients). If set, this feature looks up recipients as well as senders in the access database.
FEATURE(rbl). Enables rejection of mail based on the Realtime Blackhole List maintained at maps.vix.com.
FEATURE(accept_unqualified_senders). Normally, sendmail will not accept mail from a sender without a domain attached -- for example, user instead of user@B.NET. This feature allows such users.
FEATURE(accept_unresolvable_domains). Normally, sendmail will refuse to accept mail that has a return address with a domain that cannot be resolved using the regular host lookups (a technique commonly used by spammers). This feature permits acceptance of such addresses. Unresolvable domains can be selectively accepted using the access database.
FEATURE(relay_based_on_MX). Setting this feature permits relaying for any domain that is directed to your host.
Several other features are probably a bad idea, unless you are inside a firewall, because they make your system vulnerable to abuse by spammers:
FEATURE(relay_local_from). This feature allows relaying if the message claims to originate at your domain. Since forging this address is trivial, this is probably a bad idea.
FEATURE(loose_relay_check). This turns off checking for explicit routing through your host, such as target%C.ORG@A.COM.
FEATURE(promiscuous_relay). If you really want to forward all the spam on the net, this will turn off all checking for relaying.
Hope this is of some help.
Regards,
Clint.
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