Simply put, a bounce is a notice from the receiving ISP or domain that the email you attempted to send has not been delivered. Knowing that much can help assess general campaign performance, but it doesn't begin to tell you how to improve your results or give you the information needed to manage your list or rectify the practices that may have caused the bounce in the first place. You need to drill down into your hard and soft bounces for those answers.
The ISPs and other domains return a code and text message to the sender when an email isn't delivered. What's commonly termed a hard bounce tells you that the reason for non-delivery is due to a permanent condition, whereas a soft bounce indicates that the condition is probably temporary. You may encounter a soft bounce when a customer's mailbox is full due to vacation or even when a domain is temporarily not accepting mail because of technical problems. A soft bounce is telling you the email address is probably good but that delivery can't be completed right now. You should try sending again at a later date.
But don't assume the opposite is true with a hard bounce. A hard bounce does not automatically signify a bad (undeliverable) record that shouldn't be attempted again later on. Unlike the postal world where returned mail clearly equates to an undeliverable address, such as 'moved, left no forwarding address,' a hard bounce in email can mean many things. For instance, some hard bounces, such as a spam block, tell you that the record should not be re-tried until the underlying practice problem has been addressed. Others may relate to your technical infrastructure, such as your DNS, authentication protocol or sending speed. And still other hard bounces, such as an unknown user, signify that the record is truly undeliverable, and should be corrected or invalidated and replaced.
Without visibility into the reasons for their bounces, email marketers risk invalidating good records while keeping bad, and not addressing the underlying practice deficiencies that imperil their email deliverability, brand reputations, and bottom lines.
Source: Multichannel Merchant