Manage The Bounce Back Messages You Receive.
Soft bounce back messages usually mean there is a temporary problem delivering the email, such as a problem with the server or a full mailbox. Attempting to resend your email to the client is usually okay, because there is a good chance that your email will be sent successfully on the second or third attempt.
Hard bounce back messages are a permanent deliverability problem, such as an invalid email address. In this instance, there is no chance of the email ever getting delivered, therefore it is important to remove the email address from your database immediately.
Internet service providers (ISP’s) track the amount of bounce back messages you generate, each time you send an email. They will use this information to determine your reputation, and in some instances if you generate too many bounces, they can block any future messages.