Atomicorp

I have problems with emails not being sent to the recipients.. and not sure where to start looking for the problem.
A example... Email sent to a forum member and it is bounced back to me with the following message:
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Hi. This is the qmail-send program at name.server.com.
I'm afraid I wasn't able to deliver your message to the following addresses.
This is a permanent error; I've given up. Sorry it didn't work out.

<recipient@recipientemaill.com.au>:
Sorry, I wasn't able to establish an SMTP connection. (#4.4.1) I'm not going to try again; this message has been in the queue too long.
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Yet I can send this exact email to the exact email address from my external email account (email address that does not use my server) and it goes through without a problem.

My server provider tells me there is nothing wrong with the server.. yet emails still have issues.. which tells me there IS a problem somewhere.. just not sure where to start looking.

Server running Cent OS5, PHP5, Plesk 8.6, ASL, 4PSA Spam Guardian and Clean Server.

RBL block could do that, or straight firewall rules.

on my server??
what should I look for?

google for rbllookup sites. Run your IP's through them and see if you're listed on any