-----Ursprüngliche Nachricht-----
Von: Ralph Seichter via mailop <[email protected]>

* How and when would you decide that the recipient responding with 5xx
was due to the message itself or due to a blacklisted IP address?
That's not something that can be decided based on SMTP.

You are right. I can not rely on any sub return-code or error-message as others 
also stated here.

But sometimes the error messages are clear and also the intention of the 
recipients so one backs off.



* Can you unilaterally decide if it was acceptable to withhold the
non-delivery status from your customers?

Of corse not. However there could be an opt-in for senders so they give the 
delivery completely in other hands.

It's just a theoricaly question to see the pro/cons. And i now starting to see 
some of them.



* Are your customers OK with not being informed about your problems due
to blocked IP addresses, or do they feel misled regarding the quality
of your services?



Informing the customers of service disruption is important to stay transparent 
and something we do. However should it be primary happen by bouncing mails back 
that in most cases, are not clear to the sender or will be read at all? There 
are (also) other channels to bring it to customers attention.



* Why not solve the underlying issue of having your addresses blocked?




You know, as you are a mailop yourself, it happens from time to time. There is 
no silver bullet and for this rare cases, we are trying to deal with it in a 
more modern and better way.

Just some examples, that we have seen the last years:



- we change hostnames of our mailservers but other MX have outdated/old 
DNS-informations. I'm not talking about the TTL, sometimes anti-spam appliances 
cache DNS-informations for more than one week. Thats what we saw.

- other MX detect portsans from neighbor-networks and decide to block the 
parent-network (/23 in our case)

- other MX rely solely on geo-location of sending ip or have outdated 
informations as networks have changed owner



> I dont want to bother my users with bounces and have them send mails
> over and over again until problem is solved.

As a hypothetical customer of yours, I'd insist on knowing about every
failed e-mail, especially if it was due to problems on your end. If you
cannot address the issue that gets your IP addresses blocked, I would
choose a different service provider. A mistrusting individual might
interpret your "don't want to bother" as "don't want to risk losing".



Sure and by default, a regular bounce is and should always be the default 
behavior. But as stated earlier, it could be an opt-in option to have mail 
delivered without another sender interaction.

If this is a real benefit for the user, it's reasonable to think about making 
it an option.



-Ralph

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