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Benjamin M. A'Lee wrote: > On Thu, Jun 25, 2009 at 01:26:08PM +0100, Mick Vaites wrote: >> >> So the bounce comes back from daemon@xxxxxxxxxx to fake(s)@yourdomain.com. >> Smarthost trys to deliver this to your server and you respond "no such user" >> so the smart host sends a response back to daemon@xxxxxxxxxx "no such user". >> (Consider you could also have smart hosts at both ends ... Yuk). No, the envelope sender of NDR is "<>" so this particular kind of scatter is not a problem for a backup MX, since it will just bit bucket it anyway (or send it to the postmaster in some configurations - oh joy). The problem is spam addressed to non-existent addresses. > Um? Surely a smarthost is only for clients to send to, probably > authenticated, not accepting mail from the internet in general. I assume he means for the sending process. But the smarthost only needs ensure that people send only for the domains they should, and most not even that. Simple volume limits and alerts will prevent most issues getting out of hand. > I don’t know if there’s a huge amount of value to having a backup MX > nowadays, but I’d suggest it shouldn’t be sending a no-such-user message > back to the sending host, since this will probably lead to backscatter > if the spammer has faked the from address (of course, nobody would > actually do that, would they?). A backup MX shouldn't accept email for addresses that don't exist. If they did, they should send a NDR the RFC is very clear on this, because it may simply be misaddressed and the sender will want to know that. But accepting it in the first place is the mistake here. >> I've been following a customers exchange server overnight which was taken >> offline "no such user" responses. Yes maybe they shouldn't be building >> exchange servers that cannot cope with thousands of "no such user" messages >> -- but unfortunately there are loads of boxes out there just like this one. > > Exchange in “doing it wrong” shocker. ;) I'm not sure the relevance either, but I doubt queuing up all the misaddressed email on a backup MX, and then firing it at the Exchange box even more aggressively than the usually MTA back-off algorithm will help the situation any. -- The Mailing List for the Devon & Cornwall LUG http://mailman.dclug.org.uk/listinfo/list FAQ: http://www.dcglug.org.uk/linux_adm/list-faq.html