An ECF file is an add-in used by Microsoft Outlook, an email client. It contains information that adds extra features, such as enhanced security or increased productivity, to Outlook.

Add-ins are integrations developed to enhance the functionality of Outlook. They are developed by third-parties and can be accessed through the Office store or other websites. Add-ins may be available for a variety of purposes, such as providing access to the Google marketplace in Outlook, fighting spam, advanced email snippet managing, or adding advanced print tools to Outlook. Add-ins are installed and managed differently, depending on the version of Outlook: Outlook 2003 and older - Tools → Options → Other → Advanced Options → Add-In Manager.... Outlook 2007 - Tools → Trust Center → Add-Ins. Outlook 2010 - File → Options → Trust Center → Trust Center Settings → Add-ins. Outlook 2013 & 2016 - File → Options → Add-ins.