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Re: Email System Model

2009-05-25 00:15:56

On May 22, 2009, at 3:28 AM, John R Levine wrote:
Actually, it works pretty well in practice. The main problem with secondary MXes is that spammers send messages to them even when the primary is up (which in most cases is 99+% of the time).

This leads to the fairly obvious question: if the primary is up 99+% of the time, what's the point of a secondary? There was an era when they were useful to deal with networks that didn't have routes to some parts of the net, but I'd think that's a pretty small niche now. Unless your primary is down for a long time, days or longer, any legitimate mail will just wait and retry and get delivered anyway.

After recently examining samples of DSN messages from systems carefully selected to represent legitimate MTAs, two configurations stood out as as potential sources issuing DSNs. One being MS Exchange, presumably behind an MTA isolating Windows from directly interacting with the Internet, and a similar configuration where a filtering services like Baracuda was placed within the path of the protected MTA. This tends to indicate that the major DSN problem today is not the result of backup MX systems. While recipient hashes might be one solution, the general problem appears to be a need for a standardized way to share valid mail-box information between two systems, where each system depends upon different configuration practices.


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