If there is a problem with delivering the email to a candidate, this will be indicated in the candidate's profile. You'll see the error Could not deliver message under the Activity and Messages tab.
A permanent failure means that something is wrong with the email adress. You need to update the email adress before it's possible to try sending again.
The most common reason for this failure is a spelling error.
This error is commonly caused by the recipients mailbox being full, or a temporary error on the receiving email server. It's still possible to send new emails but the issue might persist.