An interesting aspect of online group communication is the phenomena of backchannel. Backchannel in computer-mediated communication (CMC) allows participants within a group conversation to exchange private communication which is visible only to the sender and receiver. Many existing forms of CMC provide such capability – think IRC, Skype and even Twitter (through direct messages).

Launched 5 months ago Subtextual (until recently, known as bccthis) is an interesting plug-in for Microsoft Outlook that allows the mixing of public (visible to all recipients) and private (visible only to specific recipients) content within a single email message. This allows a sender to send a single message, but add private context addressed to only those people that need it.

Subtextual adds the ability to send a hidden message as part of a normal email message. This hidden content is visible only to selected message recipients – other recipients never see any indication that the message has any additional content. Happily, recipients don’t need any plug-in to view Subtextual messages.

As well as the Outlook plugin, Subtextual also have a Twitter client (which seems less compelling to me), a FireFox plug-in for using Subtextual with Gmail and a BlackBerry application.

While clearly an interesting idea, I’m not sure whether Subtextual, is significant enough to be more than just another feature for Outlook. I am, however, impressed with their family of products across desktop, mobile and web-based email. Together with their recently announced premium version of the Outlook plug-in, it feels like the company is busy experimenting, trying to discover the platforms which can deliver them traction, customers and revenue. I am very interested to see in which direction this company will pivot in the future.