When someone uses Hide My Email, Apple generates a random email address - which is the email address you receive. That email address forwards emails to the user's original email address.
You don't say why/how the email "doesn't work", but if you're getting errors like "the email address doesn't exist", that's likely because user's are able to delete those relay email addresses.
The point of someone using "Hide My Email" is to hide their actual email address. There is no way you can get the user's actual email address - they didn't want to share it with you - and you should treat the email you have received as that user's email address.
If that email address is bouncing emails, then:
- Is a user still trying to log into your website with that email address? If so, then you should let them log in (their account credentials are still valid, you're just getting bounced emails), then inform them that their email address isn't working. This is standard practice. If emails to that user are bouncing, then you ask for an updated email address.
- If a user isn't logging in with that email, then try and send them an email after about a year saying their account will be closed/deleted in 90 days if they don't sign back in and provide a valid email address. They may or may not get the email, but at least you have a record of telling them via the only email address you have for them, that their account will be closed.