The EFX extension is given to files associated with a fax software utility called Everex eFax. EFX files are used to store the content of electronic faxes. These files are attachments to e-mail messages.

EFX files can be viewed in the free eFax Messenger software. This tool also allows you to convert this format to more popular ones, for example PDF or TIF. The eFAX software was developed by j2 Global, Inc.