A soft bounce is a message that is recognized by the recipient's mail server but comes back undelivered. This could occur because their mailbox is full, or their server is down or it is simply overloaded with incoming messages. It sometimes happens when the message is too large (another best practice - try to stay under 35K). Your email system or service should automatically retry for a few days and, if still undelivered, automatically record it as a hard bounce.

A hard bounce means either the receiving server purposely rejected the message or the server doesn't exist. Bounces denoted as "550 errors" are truly invalid email addresses - don't waste your time trying to fix them. A "Host not found" usually means there is an error in the username part (before the @) and can somethimes be manually fixed.