This article prides information about what it means when your account is bouncing.

If your email account ("jane_doe@isp.com") refuses to accept email from other accounts, then your account is said to "bounce" email. This usually means that if someone sends you an email, their message is returned to them with an error from your Internet Service Provider (ISP).

If Yahoo! Groups gets such an error back from your account after sending you a message, we consider your account to be "bouncing" email.