This is a good idea cc @zogstrip – we can send an automatic PM when we suddenly get a ton of bounces for an email that was known good, and the user can switch their email address to a working one on their own via user preferences.
The user won’t get any email about this of course, but they are (in theory) still logging into Discourse, and would see a Discourse PM about it.
We should put this on the 2.1 roadmap.