When users send documents via external mail, the sent documents can be displayed through an external link, through DocPop, or through Unity Pop files or links (depending upon your system's configuration and licensing).
To configure documents to be displayed using legacy Unity Pop files, open the Unity Configuration file and set the emailLinkAs setting to upop-file. To configure documents to be displayed using Unity Pop links open the Unity Configuration file and set the emailLinkAs setting to upop. See the Unity Client documentation for more information.