Re: SMTP traffic control

On 29/10/2011 18:26, Peter J. Holzer wrote:
> On 2011-10-24 15:24:49 +0100, Paul Smith wrote:
> > (Of course we have bigger 'pain points' - the '10 errors and we'll drop
> > you' is the worst:
> > rcpt to:<invalid(_dot_)address(_at_)bad(_dot_)domain(_dot_)com>
> > 4yz invalid recipient
> > rcpt to:<another(_dot_)address(_at_)another(_dot_)bad.domain>
> > 4yz invalid recipient
> > ... 8 more times
> > 4yz too many syntax errors, connection dropped
> "Invalid recipient" should be a 5xx error, or do you expect that these
> addresses will be created within the next few days?
Oh, I agree, but we *often* get ISP smarthost mail servers giving a 4yz 
error for 'invalid recipient'. We have to deal with this as it is our 
software which is failing to send the message, and is retrying - the 
user sees 'invalid recipient' and thinks our software shouldn't retry, 
so we have to try to explain SMTP result codes to them...
(I suspect the server is giving a 4yz because it is doing call-forward 
testing whether the remote recipient exists, and gets no response either 
from a DNS server or mail server for that address)
> > (now a message in the outbound queue which will never go)
> > but that's off topic)
> If "invalid recipient" is a 5xx error, the sender will omit the already
> rejected recipients on the next delivery attempt. Eventually all invalid
> recipients will be removed and - if any valid recipients are left - the
> message will be delivered. (And frankly, anybody who sends a message to
> tens of non-existent addresses deserves a bit of pain)
Yes, if 'invalid recipient' was a 5yz error, all would work great - but 
when the onward server incorrectly uses 4yz, it doesn't - and the 'too 
many syntax errors' connection drop (when it isn't actually syntax 
errors at all) just makes the problem much worse.
I agree that if they send to non-existent addresses they deserve some 
problems, but these are usually collected addresses for contacts which 
no longer exist, and it would sort itself out, if either the onward 
server gave a 5yz error, or didn't drop connections after treating "4yz 
recipient doesn't exist" errors as 'syntax errors'
We tend to tell the users to contact their ISP and tell them to sort 
their ******* mail server out (to which the user usually says,they'd 
love to, but don't want to spend 2 hours in a phone queue waiting to 
speak to someone who doesn't speak English and will just say 'there are 
no problems at our end'). OTOH, it sometimes gives us the option to 
resell our own SMTP relay service... ;-)