Re: [exim] The risks of analogies

Chris Lightfoot wrote:
[snip - useless crap]

There's also taking an analogy too literally.

Let's move the analogy back to the SMTP domain.

An MTA is a computer program that does not have have cognitive abilities
that enable it to read error message. It simply sees one of three
answers to each command it send in the SMTP transaction.

2XX - Command accepted, continue. Green.

4XX - Temporary failure, try again later. Amber.

5XX - Permanent failure, do not try again. Red.

It does the best it can at delivering a message and that is all it can do.

If the user who gets the error message back from his MTA that a message
he sent is unable to be delivered, then it is up to him to contact the
administrator, recipient or some other person, to fix that problem. I
recommend a phone call.

It is NOT the responsibility of his sending MTA to decipher the message
intended for a HUMAN to read and change its behaviour to break the SMTP
protocol. Period.

I have read this entire conversation and seen that all the examples
provided to not justify breaking the SMTP protocol. There is always
another way around the issue that doesn't break anything.

Incidentally, I am a cyclist. I have learned how to trigger the traffic
lights as necessary and in most cases where there is an issue with the
lights, I dismount and use the pedestrian crossing. I tend to stick to
the cycle paths however, as they are significantly nicer than inhaling
vehicle exhaust. As you can see, your counter analogy is also flawed.
Whoop-de-do.

We are not talking about a human versus a machine here - we are talking
about the MTA talking to another MTA. Upon receipt of a permanent error,
it is the responsibility of the sending MTA to inform the sender so that
the HUMAN can take the appropriate action. A MTA selectively ignoring
5XX errors is a dangerous and broken thing.