I'm new with zimbra and i did my first install (Release 6.0.1_GA_1816.DEBIAN5_64 DEBIAN5_64 FOSS edition.) following guides and wikis, well everything went good so far until i started testing the email capacity of zimbra. At first i tried sending emails to gmail yahoo and hotmail, well they arrived quickly but when i tried to send email to a private domain i can't i instantly get a message like this:

This is the mail system at host zimbra.mydomain.com.

I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to postmaster.

If you do so, please include this problem report. You can
delete your own text from the attached returned message.

The mail system
Well the hostname of my machine is zimbra.mydomain.com and i do have reverse dns and mx entries, well this server is a subdomain of mydomain.com wich is located in another machine (dunno if this has something of importance), also there is a ton of logs and i don't know which ones are those who register this type of errors.

I'm not blacklisted btw

I'm sure that i'm missing something, if you can please give me some advice i appreciate it.

If you need any extra info i will post it right away.

Thanks in advance.