Thanks for the question. That is correct DNS errors are not covered here. Let me explain how that works.
Say you send an email and the primary server is down that could be considered a DNS error since it didn't respond so say it's currently set to 3.
It would go 5, 15, 30 Bounce
If the server is currently overloaded and responds but says that the server is busy it will follow that entire interval sequence that you have.
I recommend setting the DNS errors before bouncing to the same as the number of your spool intervals to ensure delivery. The only issue with this is if the client mistypes an email and it goes nowhere they will need to wait until the spool intervals finish to get a bounce message back from the server.
I also recommend that you turn on SMTP user verification on your incoming Gateway server as it can cause more issues than it's worth like lost email on the gateway when the server goes down plus verification is already done at the SMTP level when email hits the main server.
Please let me know if this answers your question.
Technical Support Specialist