To redirect SMTP (port 25) traffic from users to a specific internal or external SMTP server, you must use dst-nat. This modifies the destination address and port to point to the desired mail server.

A.✘passthrough – Allows the packet to be evaluated by other NAT rules; it doesn't alter traffic

B.✔dst-nat – Rewrites destination IP/port; this is what is needed to redirect SMTP to a specific server

C.✘redirect – Sends traffic to the router itself; not suitable for external redirection

D.✘tarpit – Used for slowing down malicious TCP connections, not redirection

Extract from MTCNA Course Material – NAT Types:

“Use dst-nat to change the destination IP address. This is suitable for port forwarding or service redirection.”

Extract from René Meneses Study Guide – NAT Rules:

“To redirect traffic to a specific server, use action=dst-nat and specify the new destination address.”

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