/usr/share/augeas/lenses/dist/mailscanner.aug is in augeas-lenses 1.4.0-0ubuntu1.1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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Module: Mailscanner
  Parses MailScanner configuration files.
Author: Andrew Colin Kissa <andrew@topdog.za.net>
  Baruwa Enterprise Edition http://www.baruwa.com
About: License
  This file is licensed under the LGPL v2+.
About: Configuration files
  This lens applies to /etc/MailScanner/MailScanner.conf and files in 
  /etc/MailScanner/conf.d/. See <filter>.
*)
module Mailscanner =
autoload xfm
(************************************************************************
 * Group: USEFUL PRIMITIVES
 *************************************************************************)
let comment  = Util.comment
let empty = Util.empty
let space = Sep.space
let eol = Util.eol
let non_eq = /[^ =\t\r\n]+/
let non_space = /[^# \t\n]/
let any = /.*/
let word = /[A-Za-z%][ :<>%A-Za-z0-9_.-]+[A-Za-z%2]/
let include_kw = /include/
let keys = word - include_kw
let eq         = del /[ \t]*=/ " ="
let indent     = del /[ \t]*(\n[ \t]+)?/ " "
let line_value = store (non_space . any .  non_space | non_space)
(************************************************************************
 * Group: Entries 
 *************************************************************************)
let include_line = Build.key_value_line include_kw space (store non_eq)
let normal_line = [ key keys . eq . (indent . line_value)? . eol ]
(************************************************************************
 * Group: Lns and Filter
 *************************************************************************)
let lns = (empty|include_line|normal_line|comment) *
let filter = (incl "/etc/MailScanner/MailScanner.conf")
            . (incl "/etc/MailScanner/conf.d/*.conf")
let xfm = transform lns filter
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