Which Secure Email Gateway implementation method segregates inbound and outbound email?

Using a single listener for both inbound and outbound traffic does not achieve segregation; it processes all email directions through one point.

This option describes a pair of listeners that *would* achieve segregation, but it represents the complete segregated setup rather than the fundamental unit or method of dedication that option D describes.

This option describes a robust segregation method using multiple listeners and interfaces, which is technically correct for segregation, but it's a more complex setup than what option D describes as a singular dedicated listener.

To segregate inbound and outbound email, an implementation would typically dedicate separate listeners for each traffic direction. "One listener on one logical IPv4 address on a single logical interface" represents a single, dedicated endpoint that can be configured exclusively for either inbound or outbound mail, thereby facilitating the segregation of traffic flows when paired with another dedicated listener for the opposite direction.