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2011-10-28 16:31:34

(moved over from the current "SMTP traffic control" thread to stand out from 
the greylisting discussion a little)

Mail that's delivered quickly, in a minute or so, vs mail that sometimes takes 
a half hour to deliver makes a difference to the end user. Both due to the 
noticeable delay, and the annoyance of conversations arriving out of order.

Sometimes mailservers want to shed load, or prioritize one source of traffic 
over another. The only way to do that within SMTP that doesn't require you to 
keep a session open is to respond with a 4xx deferral and wait for the client 
to retry.

An ESMTP client, such as your ISP smarthost, will wait a while before retrying 
in response to a 4xx deferral. The RFCs say they should wait at least 30 
minutes the first time. 

Hence  It's 
a very lightweight approach that allows a server to tell a cooperating client 
how long it would like the client to wait before retrying. I suspect it has 
some issues that the IETF process might object to and I'm not sure that 
extending the idea to 5xx and 2xx responses is as good an idea as it seemed at 
first, but I think it's a useful concrete example to use as a basis for 

A server that's doing greylisting could certainly benefit from this, but I'm 
not particularly interested in either focusing on the implications for 
greylisting nor expanding it to do other things that greylisters might find 
useful in this thread. 


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