I think that is the fuel pump relay. The overvoltage protection relay is smaller, silver (not black) and has an ATO fuse in the top of it under a clear cover.
The OVP relay is the power feed relay for the idle control motor and the fuel control ECU. Of course it is easy to check its function with a multimeter, the problem is that since when you check it you won't be having a problem so you won't see a problem with your test. I have driven 300Es hundreds of miles with as many as five different things being monitored to solve intermittant stalls.
I have had these stalls caused by OVP relay, Fuel pump relay (not common way of failing), crank sensor, EHA (electro hydraulic actuator), speed sensor (on back of speedo), the air flow meter (the acceleration/air flap potentiometer - not available separately) and the decel micro-switch (on 88- models).
I have checked, fixed, and failed at least once a week for over ten years with these problems. Almost everyone one of the above problems would take no more than a few minutes to diagnose if they would stay broke for more than an instant.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician