A “bounced” email is one that is sent but cannot be delivered. Hard bounces are permanent failures such as nonexistent accounts. Below are some common reasons for a hard bounce:
  1. A hard bounce occurs when you send the message to an invalid account.
  2. A hard bounce might occur because the domain name doesn't exist or because the recipient is unknown.
If the bounce error is caused by a permanent failure, such as an unknown user or address, the account is customarily marked invalid by email system and will not be sent to again. You can view bounce information about these accounts within the email tool.

However, when the recipient's name is known, email may be rejected because the sender's mail box is full or for other reasons. This is known as a soft bounce.