Alessandro Vesely wrote:

Possibly, but there are others.

I think the solution you mentioned can provide the most robust MX choosing strategy we can hope for. But it strikes me as demanding a lot of work to implement and requiring us to take a lot of decisions during implementation that could leave us at bad places in case of mistakes. In other words, the best solution if well done but particulary difficult to do well.

He is an idea of mine in pseudocode:

----------------------------------------------------------------- if (number of MXs in MXList > 1) { create NewMXList from MXList (NewMXList will get only the top priority MXs from MXList) if (number of MXs in NewMXList > 1) { randomize(NewMXList) } MXList = NewMXList }

-----------------------------------------------------------------

This code could be included right after mxlist creation inside the sendesmtp function. Even before the check for broken MX records, if I am not mistaken.

I think such a solution would give us results almost as good as the ones we get from the solution you mentioned and it's certainly much easier to be done right.

This solution also has a few side-effects than seem good to me. I am not sure they are really all good so I list the ones I am aware of here so others can think about.

1. Frees Courier from the current bind dependency in terms of DNS cache used (we bring the responsability of MX randomization to Courier and we guarantee that this randomization is done in a proper way for Courier needs).

2. Provides some load-balancing for destination domains (top priority MXs will be accessed in a more random way than the current one).

3. Reduces the number of domains Courier will refuse to deliver messages to because of RFC 1035 violations (only top priority MXs will be RFC 1035 checked). I think this is ok as we are only interested in the top priority MXs anyway.

Well, there is obviously a third and much more concrete option: the one Sam effectively provided in Courier 20050626. My first impression after a quick look at it is that this solution is uneffective but I can't elaborate more on it now as I need more time to better understand it and it's implications.

Thanks for your kind words and considerations,