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On Thu, Jun 25, 2009 at 3:13 PM, Mick Vaites wrote: > In respect to the external MX regularly checking if an address is > deliverable - that is an interesting concept. How would you do this as I > thought (correct me if I'm wrong) that VRFY is disabled by default or in > some cases not even supported these days? I am fairly sure that what the external MX does is make a SMTP connection to the internal one and quits the connection right after the reply to RCPT; this reply is used to decide whether a certain address exists or not. This information is then cached on the MX, so that it doesn't have to do this check every time. I find that on our domain a few random non-existent addresses receive quite a lot of email (spam), so it would work well to block these. Of course, if you are suffering from a proper dictionary attack with many addresses that the external MX has never seen before, this will mean a lot of SMTP connections. But then, if the MX simply accepted all email, it would still have to make that connection in an attempt to deliver that email. (Of course, if the internal MX can not be reached, the MX will probably just have to accept the message.) > I'm particularly interested because at present we have a number of mail > server who run from ldap database servers. So at the edge it's possible to > confirm if a message will be delivered and where. Then on the mailbox > servers themselves again to confirm to whom the messages are to be delivered > to. The issue is that under "big load" it would be nice for the external > MX's to be able to operate autonomously. Is this what you are suggesting ? Well, I suppose it depends on what the big load is caused by: if it is caused by a lot of genuine traffic and/or 'normal' spam, then yes, using clever caching would mean the MX should be able to operate more or less autonomously. But in case of a dictionary attack, no, it will still need to verify the addresses with the internal MX. I hope that helps. Martijn. -- 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