A “bounced” email is one that is sent but cannot be delivered. A soft bounce is an email message that gets as far as the recipient's mail server but is bounced back undelivered before it gets to the intended recipient.  A soft bounce message may be deliverable at another time or may be forwarded manually by the network administrator in charge of redirecting mail on the recipient's domain. While there are many reasons an email address may soft bounce, below are some common reasons this could happen.
  1. Mailbox is full (over quota)
  2. Recipient email server is down or offline
  3. Email message is too large
On the other hand, a hard bounce is an email message that has been returned to the sender because the recipient's address is invalid.