Recently silly old me added a Secondary MX record as a backup in case the primary goes down. I figure that since the primary has the higher priority that the secondary would not come into play unless the primary was unreachable. My small screwup came about when I misconfigured the address of the secondary MX record. Now it seems we are bouncing about 50% or more of our emails. I think this is the cause because it is the only thing I can think of that I have changed.
I am seeking clarification here. I was under the assumption that only the primary MX record would be used unless it were offline. I have read that the secondary is used regardless in a type of round robin fashion along with the primary.