First of all, an Email account needs to be configured on the IP PBX with access code. Then others can send Email to this mail address with Fax file attached, and the mail title need to be the format:
access_code=fax_number
Then the built in mail server will check this account’s mail, if there’re emails with such title, the IP PBX will download the attachments and send fax to the fax number specified.