To use email2fax, simply send an email to [recipientfaxnumber]@[org].rpxfax.com (i.e. 1112223333@acme.rpxfax.com)
The email address of the sender needs to be listed somewhere in the user table. For example, if I am sending an email2fax from support@softlinx.com, I'll need to make sure there is a user in https://portal.rpxfax.com with support@softlinx.com listed as their email address.