sub checkmail {
        my $mail = shift;
        return '' if !defined $mail || ($mail =~ tr/\@//) != 1;
        my ($user,$serv);
        if ($mail =~ m/^(.+)@(.+)$/s) { $user = lc $1; $serv = lc $2; }
        else { return '' }
        $serv =~ s/\Q$1// while $serv =~ /([\n\r \])>}'";,])$/;
        return '' if
                !($serv =~ /^.+\..+$/)
                        ||
                $serv =~ /[^a-z0-9.-]/
                        ||      
                $serv =~ /\.\./g
                        ||
                $serv =~ /\.$/
                        ||
                (($user =~ tr/"//)/2) =~ /\./
        ;
        $user =~ s/""//g;
        $user =~ s/\Q$1// while $user =~ /^([\n\r \[{(<';,])/;
        return '' if $user =~ /[^a-z0-9._+&="-]/;
        return $user . '@' . $serv;
}