Dear Monks,
What is the meaning of exchange key in this module? For example in this code

use strict;
use Data::Dumper;
#########################################
die "INPUT: domain:\n" if (@ARGV != 1);
my $domain = $ARGV[0];
#########################################
# Use your own resolver object.
use Net::DNS;
my $res = Net::DNS::Resolver->new;
my  @mx = mx($res, $domain);
print Dumper (@mx);
[download]

for domain yahoo.com I am getting 3 records:

'exchange' => 'mta7.am0.yahoodns.net'
'exchange' => 'mta5.am0.yahoodns.net'
'exchange' => 'mta6.am0.yahoodns.net'
[download]

this is related to my question in http://www.perlmonks.org/?node_id=929571