> On Jan 24, 2018, at 11:32 AM, Michael Wise via mailop <[email protected]> 
> wrote:
> 
>  
> Much internal discussion, but …
> Apparently …
>  
> “ 452 is a special error code (per RFC) that instructs the sending MTA to 
> retry the rejected recipients immediately.
> “ it is designed to split messages with a large number of recipients.

http://tools.wordtothewise.com/rfc/5321#section-4.5.3.1.9

Not exactly compliant, but I guess it's a reasonable hack.

>  Apparently that’s not exactly what we’re using it for, but there you go.
> In theory, one or more of the recipients would deliver, and the first one to 
> a different region would get this response, and then the next try for the 
> remaining … should deliver.
> Unless there’s more than two other zones, in which case it’s now an N-1 
> problem, and … Lather, Rinse, Repeat until Done.

Seems like it should work, in that a compliant sending MTA should retry with 
some deliveries potentially delayed.

Should smarthosts be scheduling delivery attempts that get a 452 in response to 
RCPT TO for immediate retry, or is that a bit risky?

Cheers,
  Steve



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