What is the key difference between FortiMail's server mode and transparent mode?

What is the key difference between FortiMail’s server mode and transparent mode?

What is the key difference between FortiMail’s server mode and transparent mode?
A . Server mode only supports encrypted emails.
B . Transparent mode does not support email filtering.
C . Server mode requires additional hardware.
D . Transparent mode operates without altering the existing mail server infrastructure.

Answer: D