At 14:36 13-06-2007, Rolf E. Sonneveld wrote:
today I got the following mail from Postmaster at getronics.com
(Getronics being an ICT company with many thousands of employees,
spread over several countries):
The sending system (IP address <my MTA's IP address here>)
apparently retries delivery only every two hours. The intial retry
after a 421 status should be less than 30 minutes. Please refer to
RFC 2821, section 18.104.22.168 "Sending Strategy", where the recommended
first retry should be 30 minutes, and there should be two retries
within the first hour. The initial retry after a 421 status should
probably be somewhat shorter (say 15 - 20 minutes).
The problem here is that the greylisting mechanism used by Getronics
remembers the connection attempt for only one hour. After that the
next connection is seen as if was a new, and not a retried,
connection and it's greylisted again.
A 450 status code would have been more appropriate for greylisting.
The owner of the sending server should adjust the first two retry
intervals (and certainly the first one) to fall within the
recommendation made in RFC2821 to avoid problems when interacting
with other email systems.
Am I misinterpreting RFC2821 in that (IMO) there is no such
requirement as stated here?
There is a recommendation in Section 22.214.171.124 for the first retry to
be at least 30 minutes. There is no recommendation for two retries
within an hour.
It is not my intention to start a (flame) war on greylisting, but
keeping greylist information for only one hour needs a reality
check, isn't it? I know at least two mature and well-known MTA
implementations that have a default retry interval which doens't
match the requirements of this domain.
One hour can be quite short for such a purpose. Some SMTP clients
may retry after a few hours. Even if the retry is after 30 minutes,
there is no way to ensure that the connection will be successful.