I have found a really weird problem with my dns. I can resolve perfectly all names I have tried, except one. So, I cant send mails to that domain (but I can receive mails). When i ask directly to that domain nameserver using its IP like this:
dig @ipnumber uci.cu
works, and also seems to work for the rest of the people I know. I have contacted the site admin but he havent found any error on his side.
Im using Centos 4.4. Does somebody has any idea about whats happening?

The fact that you cannot send mail to that domain is that none of the mailservers configured is responding, according to dnsreport.
Perhaps you cannot resolve the domain because you miss the origin record in your zone file, i.e. something like that:

Excuse my ignorance about DNS topics, but why do I need such record?
Here is my zone file:

; Revision History: 11 mayo 2001 - jsoca@stgo.scu.cyt.cu
; Start of Authority (SOA) records.
;@ IN SOA ehtsc.co.cu. root.ehtsc.co.cu. (
$TTL 345600
ehtsc.co.cu. IN SOA ehtsc.co.cu. root.ehtsc.co.cu. (
2007051001 ; serial
86400 ; refresh
7200 ; retry
2592000 ; expire
345600 ; default_ttl
)
; Name Server (NS) records.
ehtsc.co.cu. IN NS prospero.ehtsc.co.cu.
ehtsc.co.cu. IN NS ns1.etecsa.net.
; Mail Exchange (MX) records.
ehtsc.co.cu. IN MX 0 prospero.ehtsc.co.cu.
; Address (A) records.
prospero.ehtsc.co.cu. IN A 200.55.150.178
; Aliases in Canonical Name (CNAME) records.
www.ehtsc.co.cu. IN CNAME prospero.ehtsc.co.cu.
mail.ehtsc.co.cu. IN CNAME prospero.ehtsc.co.cu.

Can you see something wrong there?

Is the domain in question ehtsc.co.cu or uci.cu?
Anyway if you try dnsreport for ehtsc.co.cu take a look at the last red category. It explains the reason why someone can configure a domain to resolve in a specific IP, but it's not necessary though.

My domain is ehtsc.co.cu. The conflictive domain is uci.cu and its subdomains.