To see why the email was not sent to the prospect, the marketing manager should refer to the list email send report to see if a suppression list was used. A suppression list is a list of prospects who should not receive a specific email, even if they are on the recipient list. If the prospect was on a suppression list, they would not receive the email. The other options are not likely to explain why the email was not sent, as the prospect was already on the recipient list and was mailable