At 01:42 19/03/2005, Martin Hepworth wrote:
I think the reason is that they think we might trust the secondary MX more than anything else and therefore let it through without checks.
I don't know about that. I think its more just a matter of the way the bulk mailing software works. A "normal" SMTP client will always go for the primary MX first, and only try a secondary if the primary is unreachable. Therefore nearly all your legitimate mail will go to the primary directly, unless your primary is down or overloaded and refusing connections.
On the other hand, I find that spam seems to hit the primary and secondary in roughly equal measure - so I suspect the bulk mailers just pick an MX at random rather than following the "primary first" standard that SMTP clients should follow.
The theory is probably that they can pump spam through faster if they utilize all an ISP's inbound MX machines :)
Regards, Simon
