Very late addition to this discussion, but thought I'd toss it in for future reference. Unencrypted email is permitted, given certain caveats. From HHS:
Does the HIPAA Privacy Rule permit health care providers to use e-mail to discuss health issues and treatment with their patients?
Yes. The Privacy Rule allows covered health care providers to communicate electronically, such as through e-mail, with their patients, provided they apply reasonable safeguards when doing so. See 45 C.F.R. § 164.530©. For example, certain precautions may need to be taken when using e-mail to avoid unintentional disclosures, such as checking the e-mail address for accuracy before sending, or sending an e-mail alert to the patient for address confirmation prior to sending the message. Further, while the Privacy Rule does not prohibit the use of unencrypted e-mail for treatment-related communications between health care providers and patients, other safeguards should be applied to reasonably protect privacy, such as limiting the amount or type of information disclosed through the unencrypted e-mail. In addition, covered entities will want to ensure that any transmission of electronic protected health information is in compliance with the HIPAA Security Rule requirements at 45 C.F.R. Part 164, Subpart C.