Exchange Delyaing Mails

I have an exchange 2003 server running on 2003 SBS, it was running well for around a year but recently I have been getting more and more NDR’s stating that the mail was not delievered in a timely fashion. I have done some research on this and people suggested upgrading NIC drivers which I did. I have also checked my reverse DNS which is correct and I have checked if I can telnet to the other domain which I can do. MS have suggested issues with the type of firewall in use (Sonicwall) ans given a fix which I have implemented but still no joy.

If anyone has any other suggestions please let me know as I just can not find the cause.