A System Configuration option can be enabled which will allow the deletion of a mapping:
You need to select: "Delete Mapping with all related data" option
The mapping or mappings can then be deleted. After they've been deleted it is recommended to revert this option to the default setting of unselected to prevent accidental deletion in the future.
It should also be noted that if deletion fails, there is currently no visual feedback from Migration Manager for Email Archives. The WebUI.txt may contain the following type of entry:
[2014-01-20 17:28:12Z] [ 1232] [ 68] [TRACE] [ ASPxGridView2_CustomCallback] Got command: deletemapping
[2014-01-20 17:28:12Z] [ 1232] [ 68] [TRACE] [ ASPxGridView2_CustomCallback] Got 1 keys
[2014-01-20 17:28:12Z] [ 1232] [ 68] [DEBUG] [ SafeDelete] Cannot delete Mapping Id 8 because it has stage 2 triggers. Skipping.
Providing visual feedback is something that is planned for a future release of MMEA.