Unable to send email out with exchange 5

Here is my problem. I setup an Exchange server 5.5 and I am able to receive mail from the internet but I can’t send out. I have a Pix-506 firewall and I opened the proper ports. So when I send the mail out I receive an error stating
Your message did not reach some or all of the intended recipients. The recipient name is not recognized. Too many hops.
I hope someone has run into this problem. Please give me some ideas.

Thanks,
Will0863@aol.com