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

I need to allow a certain IP to bypass the SMTP proxy on our Linux server. What is the command I would type in SSH to allow this?
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Duncan RoeConnect With a Mentor Software DeveloperCommented:
You would have to have very fancy firewall rules to block a certain IP from sending messages to the SMTP server after a certain period . And if you did block the IP, how does the testing authority determine that would in turn give a possible buffer overflow?
There are 2 ways to "block" an IP, either -j DROP or -j REJECT. The difference is that with REJECT, the caller gets error ECONNREFUSED (or you can configure for a limited number of other errors - see man iptables ). With DROP, nothing is returned: it is as if the connection request had gone to an open-circuit cable. It is up to the caller to time-out the request.
Possibly you can fix your problem by replacing DROP with RESET in the appropriate rule therefore.
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.
darrenlAuthor Commented:
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.
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