I generally try not to post or send in email anything that I would be upset to see in my local newspaper. When I really need to do so, I encrypt it tightly enough that there is little chance that anyone with the ability to break it would find it worth the cost. And when the reader opens such a note I expect them to delete and use a good scrubber to get rid of the residue -- so hopefully, it is not even on the users machine. But there are not many messages I send that are worth that effort, to me!
In addition, unless I really know and understand what "self destructing" means in this context and the algorithm used to implement it, I would be unlikely to trust it anyway!!
To give you some perspective to this poll, read this CNET article:
This e-mail will self-destruct in five seconds
Would you use this self-destructing e-mail application if it were available today?
-- Definitely, it's very useful. (How would you use it?)
-- No, I have no need to. (Why not?)
-- I would consider trying the application.
Get your votes in here:
http://forums.cnet.com/2706-21566_102-2216.html
And tell us how would you use this app?

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