Emails can bounce or block for a variety of reasons, some are IT related (rejected by server security rules), some sender error (incorrect email address), and even some receiver errors (mailbox full). 

Our mail provider, by default, prevents repeat emails to those who have recorded issues like above, preventing further emails being sent to these addresses until they're Unblocked.

If you get a notification about an email being blocked/bounced and think this is in error, go to your portal's Block/Bounce List from which you can Unblock those emails using the button in the grid.