Perl-compatible regular expressions are available in a number of configuration parameters. Domain lists can include file lookups, making it possible to support very large numbers of local domains. SMTP calls from specific machines, optionally from specific idents, can be locked out, and incoming SMTP messages from specific senders can also be locked out. Hosts that are permitted to relay mail through a machine to another external domain can be controlled by IP number or IP network number. Relay control by recipient domain and sender address is also available.
Delivery is delayed if the directory is unavailable.
Exim contains an optional built-in mail filtering facility. This can be configured to allow users to provide personal filter files, and it is also possible for a system-wide filter file to be applied to every message. The warnings can be made conditional on the contents of the message. A queue run can be manually started to deliver just a particular portion of the queue, or those messages with a recipient whose address contains a given string. There is support for the ETRN command in SMTP to interface to this.