From the mail class:

        exec { "dpkg-reconf-exim4":
                command => "/usr/sbin/dpkg-reconfigure -fnoninteractive exim4-config",
                refreshonly => true,
                require => Exec["preseed-exim4"],
        }

        exec { "preseed-exim4":
                command => "/usr/bin/debconf-set-selections /var/cache/debconf/exim4.preseed",
                refreshonly => true,
                require => File["exim4.preseed"],
                notify => Exec["dpkg-reconf-exim4"],
        }

        file { "exim4.preseed":
                path => "/var/cache/debconf/exim4.preseed",
                source => "puppet://$server/files/mail/exim4.preseed",
                notify => Exec["preseed-exim4"],
        }

From the logs:

info: Caching configuration at /etc/puppet/localconfig.yaml
notice: Starting configuration run           
notice: //example.com/mail/file=/var/cache/debconf/exim4.preseed/checksum: defined '
checksum' as '{md5}e05d3f7168c470909e5479913817be38'
info: //example.com/mail/file=/var/cache/debconf/exim4.preseed/checksum: Sums are already equal
notice: //example.com/mail/file=/var/cache/debconf/exim4.preseed/source: defined 'source' as '{md5}e05d3f7168c470909e5479913817be38'
info: //example.com/mail/file=/var/cache/debconf/exim4.preseed: Scheduling refresh of exec[preseed-exim4]
info: //example.com/mail/file=/var/cache/debconf/exim4.preseed: Scheduling refresh of exec[preseed-exim4]
notice: //example.com/mail/exec=/usr/bin/debconf-set-selections /var/cache/debconf/exim4.preseed: Triggering 'refresh' from 2 dependencies
info: //example.com/mail/exec=/usr/bin/debconf-set-selections /var/cache/debconf/exim4.preseed: Scheduling refresh of exec[dpkg-reconf-exim4]
notice: //example.com/mail/exec=/usr/sbin/dpkg-reconfigure -fnoninteractive exim4-config: Triggering 'refresh' from 1 dependencies
notice: Finished configuration run in 5.27 seconds