Exim4 implements policy controls on incoming mail by means of Access Control Lists (ACLs). Each list is a series of statements that may either grant or deny access. ACLs can be used at several places in the SMTP dialogue while receiving a message from a remote host. However, the most common places are after each RCPT command, and at the very end of the message.

Access Control Lists (ACLs) are defined in a separate section of the run time configuration file, headed by “begin acl”. Each ACL definition starts with a name, terminated by a colon. Here is a complete ACL section that contains just one very small ACL:

begin acl
small_acl:
  accept   hosts = one.host.only