Let's say we have the following messages which are all part of the same Session and were placed onto the Topic in alphabetical order.
A, B, C, D
The session receiver will pick up and process message A first before moving onto B.
But what happens if message A cannot be processed. Will the receiver automatically move onto B? Or will it still respect the sequencing?
You can find the answer in Message sessions: first in, first out (FIFO):
The session lock held by the session receiver is an umbrella for the message locks used by the peek-lock settlement mode. A receiver cannot have two messages concurrently "in flight," but the messages must be processed in order. A new message can only be obtained when the prior message has been completed or dead-lettered. Abandoning a message causes the same message to be served again with the next receive operation.
In short this means that when, in your example, message A is dead-lettered because it exceeds MaxDeliveryCount, message B will be processed.
