When using Sophos Mobile in Container Mode in Sophos Secure Email there is an option to let SSE open Attachements only in other Contrainer Apps (Sophos Secure Workspace). If you enable there only Secure Storage (to protect the Documents from other Apps) the Secure Storage isn't accessible from SSE, only Unprotected Storage would be.
So Secure Storage has to be accessible from SSE in an unencrypted way to choose attachments and send them to another internal Recipient who uses Outlook .