Email Manager for Outlook does not support External Field columns when the BCS implementation includes filters. This setting allows you to disable External Field support so it does not appear in the metadata prompt at all if your implementation has filters.
By default, this setting is set to 1 (enabled). If you set it to 0, BCS is disabled.
Enable Metadata Type External Field
7.x Email Manager Outlook6.x Email Manager for Outlook (Online-Mode Only)
|MSI Property Name||ENABLE_METADATA_TYPE_EXTERNAL_FIELD|
|Registry Root Key Priority||HKEY_CURRENT_USER|
|Registry Key Location||...\Contributor\EmailManager|
|String Value Name||EnableMetadataTypeExternalField|
|Possible Value Data||
For details on applying a configuration setting, please refer to the article How to apply a Configuration Setting.