How would I write an iptables command to allow an IP to bypass the bypass the SMTP Proxy?

This is quite difficult to answer, not knowing how your network is setup and how you are intercepting SMTP traffic and redirecting it at the moment, but something like the following line may work:

iptables -I FORWARD -s src.ip.addr. -p tcp --dport 25 -j ACCEPT

Yeah i agree with mcuk_storm, you will have to elaborate what is your current environment?

Assumption, you have a server running linux and uses iptables as Firewall.
We do not know, whether have you setup masquerading for internet access?
It will be easier if you can paste your current iptables rules here, and you can hide your public ip address.
That will ease the experts here to assist you to achieve what you want.
 

Hi guys, I apologize. My setup is a CentOS server. The reason I ask is we failed PCI compliance because I believe the firewall blocks a certain IP from sending messages to the SMTP server after a certain period, which in turn gives a possible buffer overflow. So if I could allow that IP to get an error messages back from SMTP instead of just being timed out it would let us pass.