SMTP relay

What are you trying to solve here Wil ?
I think mentioning who is your service provider will be a good starting point.

Our local ISP is etisalat UAE , we are facing DNS issues and many mails are bouced back or delay in delivery. Now we are relaying mails through our service provider where our domain is hosted and this is in USA - World link communications.

Can you post an error message that comes with the bounce back ?

Here the issue is , mails are not bouncing back and not reaching recepient. Thats the reason i am
asking whether  any possibiltiy of mails getting filtered as spam or something in relay server.

In this particular case I would diagnostic the transmission by using SMTPDIAG from Microsoft.
This tool is free to use
http://www.microsoft.com/downloads/details.aspx?familyid=bc1881c7-925d-4a29-bd42-71e8563c80a9&displaylang=en

Just expand the file on your drive. Open a command window and point to the folder you expanded it.

Then type:  smtpdiag youremail@address.com your@destination.com -d DNSSERVERIP /v

The first is your email address
The second is your destination email address
the third "-d" is the DNS IP used to solved all MX Queries
The last "/v" = verify and returns all details for each steps

You'll get a result of of this test. Copy it and paste it here.
So this way you'll have something to work with.

Also try to be as specific as possible. In one of your message you say that emails are bouncing back and in the next one, you say it doesn't. This will really help me to help you ;-)

Will i get the SMTPDIAG output as a mail or any otherway . should i do this from exchange server. I did this command from my PC. I have not received any output.

Thanks a lot , i got the output.