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A few minutes searching a legal database for sanctions involving discovery will tell the reader something. There are few things as hard to delete as an embarrassing e-mail, or worse an incriminating one. Indeed one of the major problems with document retention is that e-mails cannot be disposed of. They exist on numerous servers and if on numerous servers, they exist on numerous back-up tapes, indeed even RIM has historical copies. Moreover, one of the key features of Blackberry's etc., is that they synchonize to personal computers, so copies are on those machines, in the hard drives, on the back-ups.
What this comes down to is that someone does not want to look for the e-mails even a little, tiny bit farther than their Blackberry Inbox.
This is something that lawyers are familiar with, and recommend the use of the Mark 1 mouth and ear to reduce the related problems with. If you do not want to produce something do not e-mail it. The issue here is not that the e-mails are gone, it is that someone has an idea of what they contain and does not want to find out.