> Source: [sk98730](https://support.checkpoint.com/results/sk/sk98730)

# sk98730 - SMTP transparent proxy does not work with NAT when CoreXL is enabled

| Property | Value |
|----------|-------|
| Solution ID | sk98730 |
| Date Created | 2014-02-26 |
| Last Modified | 2017-04-12 |
| Technical Level | Advanced |
| Products | Security Gateway |
| Versions | R82.10, R82, R81.20 |

## Symptoms

- SMTP connections handled by a Security Server process are not NAT'ed when CoreXL is enabled.

## Cause

Incoming SMTP connection may be dispatched to different CPU cores.

There is a SMTP server for each CPU core (see example):  
*admin 6460 0.0 1.3 229924 53724 ? Sl Oct02 28:24 in.asmtpd 25*   
*admin 8023 0.0 1.2 229600 53368 ? Sl Oct02 22:59 in.asmtpd 25*

On the other hand, there is a single MDQ process that fetches e-mails from the spool and forwards them to the destination (see example):  
*admin 5694 0.0 1.7 279116 72752 ? Sl Oct02 40:05 mdq 0*

It always run on CoreXL FW instance 0.

This scenraio works as long as NAT is **not** involved. In this situation, we do not need the original connection that is present on another CoreXL FW instance. But, to perform NAT, Security Gateway tries to get an original connection and fails.

## Solution

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