Mark lets you mark invoices as Sent. The option to mark them as Unsent is hidden. To use Unsent, turn on Unlock Unmark in your Employee card, Access tab.
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To allow a person to use the Un-Do options in the Mark menu:
1. Go to their Employee card to the Access tab
2. Under Special Access click the checkbox for Unlock Unmark
3. Click OK to save changes to their Employee card