well, efax.com is a real site that allows people to receive faxes that are converted to images and emailed to them. The best way to tell would be to see what the source IP of your connections are coming from for those emails and see if they correspond with the efax.com domain. If they do, you are looking at probably legitimate emails. If not, then more likely, they are spam.
I've seen spam in the past purporting to originate at efax when in fact it's from eslewhere. If you aren't a customer of theirs you can pretty-well guarantee that it's spam.
eFax is a leading Online Fax service.
If you have an eFax account, you'll receive email notifications from eFax with a fax attachment in PDF or EFX format.
If you don't have an eFax account, and/or you receive an email which contains a link to a download instead of a PDF attachment, it is likely spam sent by others who imitate the eFax service..