Right clicking on a specific carrier will provide a context menu with four options.
Selecting this option allows you to requeue a range of previously delivered messages for the selected carrier.
Selecting this option allows you to cancel all past due, pending and/or repeating messages for the selected carrier.
Selecting this option will display a list of all messages in the pending queue for the selected carrier.
Selecting this option will remove the carrier from the list. This cannot be done if there are recipients still tied to this carrier.