Exchange Server 2003 allowing some e-mails in or out and not others

I recently installed exchange server 2003 on my SBS 2003 machine and I discovered that some e-mails are being sent out while others are stopped and vice versa. I suspect that this could be a DNS issue. My ISP assures me that things on their side are correct and my firewall tech support says that it is not the firewall (sonicwall by the way) I ran smtpdiag and it says that my mail can get out. I’m really at a loss and about to wipe the server and start over. I hope that someone can help me with this problem and I thank you in advance for your help.