You can view and fix email delivery failure errors that occur when a new user account is created, or any of the following changes are made to an existing user account:
Added an account
Password expiration notification
Changed password confirmation
A red flag appears at the top of a page when an email delivery failure occurs.
(Click to view image)
Click the flag to reveal a list, and select Email Delivery Failures. Alternatively, from Setup, select Users. To find users with email failures, select Only Users With Email Failures under Toggle secondary filters.