In order to avoid a high bounce rate, we use a third-party service to identify whether the address is valid or non-deliverable (can't receive email messages).
Some of the reasons an email address can be marked as non-deliverable are:
Email is no longer active, the person might not be with the company anymore;
The email is coming from a disposable domain;
The domain for email does not exist or has no valid DNS records;
The specified email doesn't have a valid email address syntax;
The email address was rejected by the SMTP server, or the email address does not exist;
An unknown error has occurred.
If needed, please allow a couple of hours, then you can re-check the address. However, please keep in mind that this will consume a survey credit. Check the screenshot below to learn how to recheck an email address from the contact's profile: