or if you have unequal sized groups of domains then this may suit:

use warnings;
use strict;

my %domains;

while (my $line = <DATA>)
  {
  chomp $line;
  my ($name, $domain) = split /@/, $line;
  
  $domains{$domain} = [] if ! defined $domains{$domain};
  push @{$domains{$domain}}, $line;
  }

my @counts;
my $avg = 0;

for my $domain (keys %domains)
  {
  my $count = scalar @{$domains{$domain}};
  $avg += $count;
  push @counts, $count
  }

$avg /= @counts;

my @ballanced;

for my $domain (keys %domains)
  {
  my @names = @{$domains{$domain}};
  if (@names <= int $avg)
    {
    push @ballanced, [@names];
    next;
    }
    
  my $toIns = @names / int $avg;
  my $inc = int (@ballanced / $toIns);
  $inc = 1 if $inc < 1;
  my $index = 0;
  while (@names)
    {
    my @sublist = splice @names, 0, $avg;
    splice @ballanced, $index, 0, [@sublist];
    $index += $inc;
    }
  }

my $going = 1;
while ($going)
  {
  $going = 0;
  
  for my $addrs (@ballanced)
    {
    next if ! @$addrs;
    my $name = shift @$addrs;
    print "$name\n";
    $going = 1;
    }
  }
  
__DATA__
ads1@domain.com
ads2@domain.com
ads1@domain1.com
ads2@domain1.com
ads1@domain2.com
ads2@domain2.com
ads3@domain2.com
ads4@domain2.com
ads1@domain3.com
ads1@domain4.com
ads2@domain4.com
[download]

Prints:

ads1@domain2.com
ads1@domain4.com
ads3@domain2.com
ads1@domain.com
ads1@domain3.com
ads1@domain1.com
ads2@domain2.com
ads2@domain4.com
ads4@domain2.com
ads2@domain.com
ads2@domain1.com
[download]