Brian Evans - Postfix List wrote:
> Mikael Bak wrote:
>> Santiago Romero wrote:
>>   
>>> Really, reject_unverified_recipient feature is very nice, but rejecting
>>> all mail when primary MX doesn't answers breaks it for us :(
>>>
>>> Any idea? :?
>>>     
>> Hi,
>>
>> Quoting the documentation[1]:
>>
>> "The unverified_recipient_defer_code parameter (default 450) specifies
>> the numerical Postfix SMTP server reply code when a recipient address
>> probe fails with some temporary error. Some sites insist on changing
>> this into 250. NOTE: This change turns MX servers into backscatter
>> sources when the load is high."
>>
>> So you are not rejecting any email if the MX is down. You are just
>> delaying reject or accept until the MX is asked if there is such user or
>> not. We're very happy with this over here.
>>   
> 
> No, you are not "delaying reject".
> You are bouncing and possibly BackSattering because you really don't
> know if the recipient is valid.
> 
> Many, many envelope recipients are forged these days.
> So you end up bouncing to the wrong place and sending spam to a 3rd party.
> 
> A good MTA in the world will hold a 450 for 3 to 5 days and keep retrying.
> If it doesn't retry, it's usually a bot and bad for your health.
> 

Hi Brian,
Well, thank you for sharing this with me.

IMO this setup does not bounce as you say, it sends a "450 Address
verification in progress. Try later.". When the client tries next time
there is either an OK the address exists, or a 550 User does not exist.

Maybe I don't understand what you try to say. I just don't see why this
would generate bounces or backscatter.

Mikael

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