WHat does 550 Currently Sending Spam bounce back email message mean?

That means that someone got a virus in you network and started spamming from your exchange server address.  You've been listed as a spammer and will be blocked by organizations using spam filters.

Once you have cleaned up the problem you can go to those spam lists and request to be taken off.  

Here's a place to start:
http://www.mxtoolbox.com/

type in you public IP for your email server.  It will check against spam filters.  If you are listed on one the lists you can click on it and request to be removed.  Be careful because they only do it so many times so make sure you environment is clear of any spamming.  

let me know what you come up with.

my network? or the recipient's network?
the recipient's network is cxa.com

thanx.
> someone got a virus in CXA network and started spamming from CXA exchange server address.  You've been listed as a spammer and will be blocked by organizations using spam filters.

I understand that cxa will have problem sending out emails as they will get blocked by organization using the spam filters.

But in my case, I am sending email to CXA, why I got this bounce back message?

Did you get any other response from thier mail server like a challenge email?  You might try sending an email twice in a row.  

See this article about greylisting:
http://www.greylisting.org/

Ah!! actually I was wrong. It was the user from CXA trying to send us the email.