EFX file extension

eFax Document

Document file created with eFax, a fax service.

These .EFX files are used by eFax to both send and receive faxes. If receiving a fax, an email is received to a user's email inbox with an attachment which contains the body of the fax. If sending a fax, the user attaches an .EFX file to an email and sends through to eFax - who then strip the attachment and send the fax to the number in the header.