well, i need to do this to feed a mailinglist into Mail::Bulkmail -- which can send emails within 1 envelope to each machine... so the machinename.domainname.tld works in this case anyways, i noticed a way to make it even slightly faster (about 8%)...

%domains = {};
for (@emails){
    @email = split /@/, $_;
    if (!$domains{$email[1]}){
        $domains{$email[1]} = [];
    }
    push @{ $domains{$email[1]} }, $email[0];
}
foreach $domain (sort keys %domains) {
    foreach $recipient ( @{ $domains{$domain} }  ) {
        push @sorted, ( $recipient . "@" . $domain );
    }
}
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