said by hamburglar_:
Unless you have sensitive information in the documents. Once they are in your email, they are much less secure.
There is no reason why you can not encrypt the eFax file before mailing it as an attachment. The major problem with emailing a Fax file as opposed to doing a direct Fax-Machine to Fax-Machine connection is that you have no way to know that the message arrived. This is the same "problem" as sending ANY email message - you know it was sent but have no way of knowing if/when it arrived.