Why am I seeing error 552 5.3.4 in my NDRs?

From time to time you may see the following error in a Non-Delivery Receipt (NDR) message:

Remote host said: 552 5.3.4 Message size exceeds the fixed maximum message size

This means the message that was attempting to be journaled exceeds the maximum size the sensor will receive. Currently, on-prem sensors will only accept messages 100mb and under. 

Reference:

Message size limits