Gordon Messmer wrote:

I will try to explain this another way.

As far as I can tell, bind randomization gives each MX a 10% chance of being the first one in the list. Up to now we have a good enough distribution.

Then Courier gets these answers and discards all MXs except the top priority ones without changing the order they were originally received. This seems ok as Courier is only interested in the top priority ones.

The problem is that when he does that it ends 9 out of 10 times with brsmtp02.br.abnamro.com before brsmtp04.br.abnamro.com which means Courier will try to contact brsmtp02.br.abnamro.com most of the times in case of a temporary delivery failure.

If my arguments don't convince you, please try it at a bind DNS cache. Query it several times for these MXs. See which MX server Courier will try to contact receiving each answer. Just remember that Courier is only interested in the top priority MXs (priority 10 in this case). It might be easier to spot the problem this way. I have done it.

I agree completely. It isn't bind randomness strategy per si that creates the distortion. Nor is Courier's MX choosing strategy either. It's the *interaction* of bind randomness strategy and Courier MX choosing strategy that produces the distortion.

I hope I made myself clearer now.

Thanks again for your attention and considerations,