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 which contains just one very small ACL:
begin acl small_acl: accept hosts = one.host.only
You can have as many lists as you like in the ACL section, and the order in which they appear does not matter. The lists are self-terminating.
The majority of ACLs are used to control Exim's behaviour when it receives certain SMTP commands. This applies both to incoming TCP/IP connections, and when a local process submits a message over a pipe (using the -bs option). The most common use is for controlling which recipients are accepted in incoming messages. In addition, you can also define an ACL that is used to check local non-SMTP messages. The default configuration file contains an example of a realistic ACL for checking RCPT commands. This is discussed in chapter 7.
The -bh command line option provides a way of testing your ACL configuration locally by running a fake SMTP session with which you interact. The host relay-test.mail-abuse.org provides a service for checking your relaying configuration (see section 38.27 for more details).
In order to cause an ACL to be used, you have to name it in one of the relevant options in the main part of the configuration. These options are: