Count me as another vote for S/MIME or PGP/MIME Support in the Webmail client.
Although email may be currently encrypted while in Transit via TLS/STARTTLS it is not encrypted while at Rest which is what is needed for HIPAA Compliance.
Right now, all of our customers who either require or desire full email encryption we are setting up with Outlook, eM Client, Thunderbird, or Apple Mail as they all support it when sending & receiving through Smartermail, but we make it clear that they can neither send or read encrypted mail in the Webmail if they opt to use encryption with these email clients. That's good enough for the majority of our customers, but a major disappointment/deterrent to others as Office365 offers this natively in their Webmail.