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Re: [ietf-smtp] Reject messages on backup mail exchangers when primary MX is online

2013-03-04 00:31:09
At 07:48 -0500 on 02/27/2013, hector wrote about Re: [ietf-smtp] Reject messages on backup mail exchangers w:

Your proposal/suggestion can hurt a working system IMO.  It can't simply
assume that the first MX will be available at all times or that it will
be the first IP tried at all times.  The specs may imply the caller MUST
sort and by implication call the first IP, but it would seem odd to me
to design anything around that.  It would be so unreliable.  You can't
enforce it or guarantee that this order is persistent or consistent.

One addendum. With gray listing, with the first attempt automatically getting rejected with a Try Again status, a good queuing method should be designed to be aware that such a Try Again response COULD be a sign of Gray Listing and compensate for it. This means that if the initial attempt is made to a MX pool (ie: There is more than one MX at the lowest priority) to record which IPN was used and do the retry to it not some MX randomly selected from the MX pool. Otherwise when you end up with a different MX IPN, it may blow you off again due to Gray Listing (ie: There is no way to know if the MXs share a common list of messages that have been Gray List blown off on the first attempt so they will accept the second attempt even if it is made to a different MX or if each have their own list so you will keep getting blown off until by chance you finally connect with a MX who has already blown you off and is thus willing to accept you this time).
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