Any law firm or lawyer engaged in email marketing will have encountered the terms "hard" and "soft" bounces in their campaign reports. What do these terms mean?
Briefly, when you distribute an email to a subscriber list, the email will not reach all of its intended recipients. When your mail server receives a message back from a recipient's mail server that the email was not deliverable, the email has "bounced."
A "hard" bounce is an e-mail message that has bounced because of some "permanent" problem - typically, because the domain used in the address doesn't exist, or more likely because the recipient's address is unknown at the specified domain. In some cases, receiving mail servers may "hard bounce" all emails received from a particular sender as spam.
With some email service providers, an email address may be removed from your distribution list after a single hard bounce. However, the "better practice" is to wait for at least 3 hard bounces separated by more than 15 days.
A "soft" bounce is an email message that has bounced due to some "temporary" issue such as a mailbox that is full, a recipient who is on vacation or "out of office", or a temporary connection failure. Email service providers will typically attempt to resend a "soft bounced" email at a later time.
For further technical information on bounces, see this entry on bounced emails on Wikipedia.