[Exim] Problem with sender verification

Hi there:

I have one problem with sender verify, from domains that are aliases of
my server, receipt a message saying that it cannot verify sender.

I have a configuration like the following one:

In EXIM (2.05)
localdomains: domain.com:alias.com
no_sender_verify

In DNS
alias.com     preference = 10, mail exchanger = mx.domain.com
mx.domain.com      internet address = xxx.xxx.xx.x

And when I run exim -d9 -M messageID.....

--------------------------------------------------------------------------
Connecting to target.domain.com [xxx.xx.xx.x] ... connected
SMTP response timeout = 300
SMTP<< 220-target.domain.com Smail-3.2.0.101 (#1 1998-Feb-16) ready at
Wed, 29 Dec 1999 12:18:28 +0500 (GMT)
220 ESMTP supported
SMTP>> EHLO source.domain.com
SMTP response timeout = 300
SMTP<< 250-target.domain.com Hello source.domain.com (source.domain.com
from address [xxx.xxx.xx.x]), here's what we support:
250-EXPN
250-SIZE 3072000
250-PIPELINING
250 HELP
use_size=1
SMTP>> MAIL FROM:<email@???> SIZE=1383
SMTP response timeout = 300
SMTP<< 450 defer '<email@???>SIZE=1383', sender address target
'alias.com' cannot be verified at this time: verify_sender(): BIND server
failure: Nameserver: Server failure: Error 0. (Try again later)

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Somebody can help me?