Hard Bounces

• If an email bounces because your recipient's address was invalid, it's known as a hard bounce. This can be the result of mistyping an email address or using an address that's no longer valid. Hard bounces happen when the domain name is nonexistent or the recipient's name is unknown.
  
  

Soft Bounces

• If an email reaches your recipient's mail server but bounces back before it can be delivered, it's known as a soft bounce. This kind of bounce usually happens when the recipient's mailbox is full.
  
  

Relay Problems

• As a means of winnowing out spam, Internet service providers generally require that a recipient's mail server must know the sender's mail server. Relay problems that culminate in bouncing email arise when Internet service providers disagree on what it means to "know" a sender's mail server.
  
  

Temporary Problems

• Mail servers can experience temporary glitches, resulting in bounced emails. These result in error messages which notify the sender that a connection has timed out or that resources are temporarily unavailable. These problems tend to resolve themselves.