The temptation of selective sender address verification
You could say that we have at least three mail problems. We have a requirement to accept all email by default, we have people who either forward all their mail to places with more strict spam policies or have autoreplies, and such people sometimes get email from known domains that can't be sent email (sometimes in annoying ways). The result is a moderate amount of email camped out on our system that I know will never be delivered, and a sender address verification temptation.
There all sorts of reasons why general callback-based sender verification is a bad idea; people much more articulate than I am have written lots about this. I would never do general sender verification. However, the temptation is that I could make our mailer do selective sender verification, verifying only addresses at certain domains. Say, the known domains that send us stuff that then causes bounces or autoreplies that said domains never accept back. This wouldn't make callback verification any less of a bad thing but on the other hand I would only be applying it to domains that are already behaving antisocially towards us (as far as we're concerned). In theory , this would have the net effect of blocking email from those 'bad' domains but (mostly) only to the extent that they themselves are being bad SMTP citizens; if they became willing to accept their email addresses back, we'd become willing to accept their email.
(In theory this is what happens with all uses of callback sender verification.)
Sadly, this is a bad idea and illustrates a certain dangerous mindset that I can get into when I think about spam. Spam fundamentally irritates me; it just gets under my skin and acts as one of my hot buttons. It's uncomfortably easy for me to slip into a mindset where I start losing balance and perspective, one where the most important thing is blocking spam and other concerns are decidedly secondary.
In theory I could argue that we have a defensible technical reason for insisting on callback verification for these domains; after all, our email system effectively chokes on these undeliverable addresses. In practice I know full well that the mail system is not being particularly affected by the current or likely future level of delivery attempts to these domains and that the real reason I want callback sender verification for these domains is that it'll cause us to stop accepting their email. We have no mandate to stop accepting email from domains I don't like (rather the contrary), so I'd be trying to sneak it in through the back door in a vaguely deniable fashion.
(Having written this, I am hopefully now somewhat better prepared to resist this particular temptation. But I have to say that it's very attractive to the bit of me that likes poetic justice and hates spam.)