Re: An approach for reducing mail loss with greylisting

On  13 Nov 2006, at 2:10 PM, William Blunn wrote:

> Ronald Oussoren wrote:
>> All MTAs should be able to deal with this, you can already get 4xx
>> status codes due to full mailboxes and other transient failures.
>
> Sorry, no.
>
> All (legitimate) MTAs should be able to deal with transient  
> failure, period.
>
> The fact is that they don't.
>
> Moving your transient failure indication after the end of DATA  
> certainly
> isn't a panacea for broken MTAs.
>
> Not only that but I don't even think that moving there would give any
> overall benefit versus tempfailing before DATA.
>
> Indeed I have heard tell that some (broken) MTAs ignore errors  
> after the
> end of DATA.

There are definitely MTAs that don't handle 4xx errors at the  
traditional place to do greylisting. I've seen both servers that  
bounce immediately (e.g. treat the 4xx as a 5xx error) and servers  
that completely drop mail. That was a major reason for dropping  
greylisting, although I'm interested in trying again now that  
greylisting has matured because greylisting was very effective at  
dropping unwanted mail.

My gut feeling is that MTAs will be better at coping with 4xx errors  
at the end of the DATA command because that's where you traditionally  
get those when mailboxes are full.

Ronald