Client Restrictions

Client restrictions define how the machine accepts mail. It defines what machines may connect to this server and send or relay mail using the SMTP service. The client restrictions section provides three policy levels: none, moderate and strict.

None

If set to none, no client restrictions will be implemented. All connecting mail servers will be able to send mail to this server if the destination is valid for this server. This option should almost never be chosen.

Moderate

Selecting Moderate rejects mail if either the sender domain or the recipient domain is not a FQDN (Fully Qualified Domain Name) or cannot be resolved by DNS. It will also reject mail if the sender hostname is in invalid format. This is the recommended option.

Strict

Selecting Strict will set the mail server to reject all incoming mail where the sender's hostname cannot be resolved by DNS, in addition to all other restrictions at the moderate level. This setting may reject valid mail under certain circumstances and must be used with caution.

At this stage, you should have set up the mail server with user accounts, assigned it a hostname and optionally defined a relay host. At this point, the machine should be able to send mail out to the Internet. The following part of this guide will discuss how to configure this mail server to accept mail for a particular domain, or to forward mail for certain domains to other machines.