return _result(CU_SMTP_UNREACHABLE,
                       'Cannot connect SMTP servers: ' .
                       join(', ', @mservers));
...
sub _result($$) {
    my($code, $reason) = @_;
    my $ok = 0;
    $ok = 1 if $code == CU_OK;
    $ok = 1 if $code == CU_SMTP_UNREACHABLE;
...

##</code><code>##

if (Mail::CheckUser::last_check()->{code} == CU_SMTP_UNREACHABLE) {
  # can't reach server
  ...
}