Agreed, this isn't functionality most board owners think about, but it's critical if you're serious about growing your community, and can become a massive administrative headache as your board scales.
It was a nightmare for me (my board sends an average of about 5,000 transactional emails per day) until I started sending email using SendGrid's SMTP service. They work with ISPs to remove you from blacklists and provide a ton of other features I've found very useful, but the best is their
"Event Webhooks " API. It's basically what @
BamaStangGuy mentioned on the last page.
When their service determines that a message has bounced or been marked as spam, they send an HTTP request to the URL of your choice. I wrote a custom little script for my vB 3.8 forum and pointed the SendGrid API to it; it just gets the bounced email address from the POST, looks up the user in question, and moves their account to the "unconfirmed email" user group.
I'm afraid the wild variations in server email configurations and services will hinder any progress on bringing something like this to the core, and will definitely be re-creating this script when I move my forum over to XenForo. If there's a lot of interest in this (as there should be for such important functionality), it's something I would consider packaging and sharing as a plugin.