On Wed, Oct 28, 2015 at 02:14:23PM -0400, Ted Lemon wrote:
And this is because the SMTP store-and-forward model requires that error
messages be delivered asynchronously, which is another architectural
problem with SMTP.
1. There is no requirement to always send asynchronous bounces,
most properly operated mail systems strive to reject
synchronously rather than accept and then bounce. Avoidable
backscatter is frowned upon.
2. It is not possible to *guarantee* delivery of all mail accepted
for onward relaying. *Some* asynchronous errors are unavoidable.
If error messages could be delivered at the moment of transmission rather
than later, at least most of the time, this wouldn't be an issue.
Most of the time, on properly configured receiving systems, errors
are already synchronous.
It's entirely possible to do it, but requires a little more
sophistication than Postfix or Sendmail have.
My future-gazing crystal ball is out of stock on Amazon, delivery
estimate not available. With no crystal ball to predict the future,
mail queued for onward relaying might later bounce.
--
Viktor.