Blackbaud NetCommunity will automatically exclude hard bounces email addresses and invalid accounts from your bulk email or email blast. Hard bounce email addresses are stored in Invalid Accounts in your Blackbaud NetCommunity administration or back-end. These do not have to be removed from the list manually.

For more information:
How long does Blackbaud NetCommunity attempt to deliver emails they are considered bounced? 
What is a hard undeliverable email (hard bounce)