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Re: Question on retry strategy in 2821

2007-06-13 17:44:04

Hi Rolf,
At 14:36 13-06-2007, Rolf E. Sonneveld wrote:
today I got the following mail from Postmaster at (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 "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 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.

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