Where can I find requirements on what to do if the bank sends a notice, disclosure or alert to a customer's email and the email bounces back? Is there still a requirement to mail?

The Fed had such a standard for the consumer protection regs it administered pre-CFPB. It was a stupid idea and the agency finally got rid of it.

If snail mail bounces back, what do you do? I recommend you do no less here. You could do more and revert to snail mail, especially if this is an important message or escheat issue. "Important" may be up to you as in what is the mailing about, income from this relationship, etc.