All fax apps are incredibly safe and use encryption to ensure your transmissions are protected. Faxing is one of the most secure forms of communication.
However, SRFax has the highest security protocols out of any online fax app. Their primary focus is providing a secure fax service for healthcare professionals, which needs to comply with HIPAA.
SRFax has strong encryption, spam blocking, user authentication, automatic fax deletion, and more. Basically everything you would expect in an enterprise plan.