The following snippet can be insterted early in the "acl_check_rcpt" ACL
in a standard Debian Exim4 installation.  If you have split your Exim4
configuration into several files inside /etc/exim4/conf.d, this should
go into /etc/exim4/conf.d/acl/30_exim4-config_check_rcpt.

Alternatively, you can use the hook mechanism built into the default
Debian Exim4 configuration: save these snippets as something like
/etc/exim4/acl_check_data_hook and /etc/exim4/acl_check_rcpt_hook and
then in /etc/exim4/conf.d/main/00                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         