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Re: [ietf-smtp] SMTP Retrying/Sending Strategy on 452 / 4.5.3

2019-02-11 10:45:56
On 11/02/2019 16:09, Hector Santos wrote:

However, problematically, a server issuing a "retry=00:00:00" hint would translate to a no block time delay and the client can try again when it decides to try again.

The key point is that a server issuing ANY 'retry=' means that the client can try again when it decides to try again. That is even if the client understands the 'retry=' hints. It's a *hint*.

The client may look at "retry=00:05:00" and say "that's too short, I'll try again in 20 minutes, or at "retry=04:00:00" and say "that's too long, I'll try again in 1 hour".

The client is what determines when it will retry. Basing any scheme around the server being able to control that is doomed to failure.

Using the hints for greylisting isn't going to be problematic because greylisting is usually solved by a single retry in the correct timeframe. Subverting them for other purposes (which may involve multiple retries, even after the specified delays) may give unintended results.

If you're on your 20th retry and the server is STILL saying 'retry=00:00:01', for the 20th time, then it wouldn't be unreasonable for the client to say 'stuff that, I'll leave it a few hours and then try it again'.


Paul Smith Computer Services
Tel: 01484 855800
Vat No: GB 685 6987 53

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