HIPAA does not explicitly require emailed PHI to be encrypted. HIPAA requires "reasonable" protection/consideratoins etc. when emailing PHI.
HIPAA does require secure transit of PHI, which could easily be violated (e.g. using a webmail client over http vs. https).
DIRECT, as mentioned by Freiheit, is a direct exchange protocol on the uptake that not only ensures the proper recipient, but authenticates the sender as a trusted source as well, through pre-established circles of trust, so to speak.
Other basic services such as Zix (very straightforward secure email service that basically emails your recipient a message "hey, some stuff was sent...click here and prove it's you [login to secure server] to read it).
If it's a theoretical debate on your thread, you can say "technically it's legal". If it's in consideration for building a workflow to share that reason, choose another path.
Some decent opinions/descriptions: http://blogs.hcpro.com/hipaa/2010/01/phi-in-e-mail/