This screen shows a list of all failed emails. If "Resend Failed Emails" has been checked in Options, then MARS will keep retrying the email until it is manually removed from the queue. As an example , if an email address has been misspelt (.cmo instead of .com). The queue will shows when the last attempt was, and what the result was.
Send Now: Select a message, and click this button to attempt an immediate delivery.
Remove Selected: Select a message and click on this button to delete it from the list.
Remove All: Click this button to clear the queue.
Only send one email alert per email failure: This works in conjunction with the "Try resending failed emails" in Options (Tools > Options). If a message cannot be sent and MARS is configured to notify you of any email failure this makes sure an alert is sent to you only once and not every time the email fails due to the configured retry.