In their November ClickZ column, Kirill Popov and Loren McDonald explained why processing bounces is important. Still, there's plenty of confusion surrounding soft and hard e-mail bounces.
Most commonly, a soft bounce is a temporary condition, such as 'mailbox full.' A hard bounce is permanent, 'user unknown' being one good example. These definitions make intuitive sense. Many e-mailers recommend ignoring soft bounces. They only process hard bounces. This, too, makes intuitive sense.
If the condition is indeed permanent, a soft bounce will eventually become a hard bounce. The recipient's mailbox will be closed, and 'mailbox full' will replace 'user unknown.' This all seems logical and straightforward.
In the real world, things are more complex. There are too many scenarios under which the above definitions can lead to misidentification and unprocessed bounces.
Read the full ClickZ' article: E-Mail Bounce Management, Simplified.