What is the correct action for a NAT rule that should intercept SMTP traffic?

In the context of a NAT rule that is intended to intercept SMTP traffic, redirecting is the appropriate action because it allows the system to send traffic intended for a specific destination (in this case, SMTP traffic) to a different destination without the original client being aware of this change. This is especially useful for scenarios such as traffic management or filtering, where you want to direct traffic to an alternative service or monitoring setup, perhaps for logging or filtering.

When SMTP traffic is redirected, any connection attempt to the designated SMTP server can be intercepted and handled according to your specific network requirements, such as filtering or passing through a mail server that performs additional checks. This allows for more control and management of the traffic flow.

Other options do not fulfill the same purpose as redirecting. While tarpit is often used to slow down or deter unwanted connections, it does not facilitate interception of traffic in a productive manner. Dst-nat typically implies that the packet is translated to a different destination but does not necessarily achieve the desired interception if it lacks accompanying rules for processing those packets. Passthrough can imply that traffic flows without alteration, which does not provide the interception mechanism needed in this scenario.