Two servers stacked with no gap (metal-to-metal).

Heat transfer occurs in three modes: conduction, convection, radiation. Conduction is transfer by direct contact of solids. Since the servers are touching metal-to-metal, heat flows directly from the warmer surface to the cooler one via conduction.

Radiation occurs without contact, across air or vacuum.

Convection requires a fluid medium like air, which is absent between surfaces.

“No heat transfer” is incorrect—there will always be transfer.

Therefore, the unwanted transfer here is conduction.