Your bank tracks each transaction that you have submitted and will notify you of any transactions which was unable to be completed.
 
For example, if an account has been closed, or if you are debiting funds and the account balance is insufficient, you will be notified by your bank. This report can be printed/faxed or can be sent electronically in an ACH Return file. Regardless of the format, the returned items will contain a standard three-digit return code (an 'R' and two numeric digits).
 
In addition, your bank may offer optional services which will allow you to track all transactions, rather than a negative-confirmation scenario.
 

See Returns for more information.