> Source: [sk122892](https://support.checkpoint.com/results/sk/sk122892)

# sk122892 - Windows NLB multicast server cannot fetch mails from behind 600/700/1100/1400 device

| Property | Value |
|----------|-------|
| Solution ID | sk122892 |
| Date Created | 2018-02-11 |
| Last Modified | 2018-02-12 |
| Technical Level | Advanced |
| Products | Spark Firewall (Locally Managed) |
| Versions | R82.00.X, R81.10.X |
| Platform | 6, 20, 400, 110 |

## Symptoms

- When Windows Network Load Balancing (NLB) cluster is configured in Unicast mode, the clustered hosts can ping and internet access outside, but POP3 connection is dropped by SMB Appliance and emails cannot be fetched.

## Cause

The SYN-ACK is sent from the gateway, during the fold process ("transparent proxy") the gateway "impersonates" the mail server, so it uses the source IP address of the Mail Server. However, it still uses its own MAC address, and not MAC address of the Mail Server.

## Solution

This solution requires authentication. Please log in to view the full solution.

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