> I'd argue this is the fair and correct behavior.
A behavior that breaks how the mail system has been working since forever, and that people expect and use all the time.
It may not be right, but it is used and not even the big players (such as gmail) are willing to break it (hence why gmail actually tells other admins to use pre-delivery forwards disregarding SPF, but respecting DKIM wich is broken if you use post-delivery aliases) [1].
[1] https://support.google.com/mail/answer/175365?hl=en
> Sounds like a misconfiguration.
A misconfiguration you cannot fix (because it is on the receiver end, not on your end). But the client pays you for the service, and understandably asks you for a solution. What would you tell them?
I understand your points, and mostly agree with them. However, this approach only works in an ideal world where users understand that e-mail should have some limitations it hasn't had for the last 30 years, and all administrators are "good citizens" (they know their stuff, acknowledge their issues and work to quickly fix them).
The real world is different: clients will demand solutions, and other admins will oftentimes be either ignorant, powerless or even adversarial.
