More information on SMTP errors:
This is a log of errors from your account as a result of using the SMTP relay. Examples of errors include notification of emails not sent due to the address being on your account's Unsubscribe or Bounce list. Errors can also include errors notifying you your account is over-quota. In most cases, errors never need to be reviewed in Reporting, as critical errors will also be emailed to the account holder. For those users that like to keep their account's data clean and organized, this new option helps better manage your SMTP errors.