Your expectations do not match those of the module's author: 
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;
...

[download]

The module's author apparantly decided not to assume a bad address if the server cannot be reached. You'll need to add your own test, i.e.

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

[download]

In reply to Re: Mail::CheckUser issue by keszler
in thread Mail::CheckUser issue by neilwatson

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