Looking for some insight into any possible systematic way this could have happened.
A confidential emailA was sent from userA to userB . UserB is using webaccess. UserB is drafting another emailB to userC and bcc'd to 8 others. IT gets call from UserA saying she is receiving messages from some of the other 8 that they received confidential emailA in error. A copy of the message sent in error has userA in From box with UserB in brackets. IT changes userB password and logs into account via web access. On login an email recovery box pops up, listing 2 emails. Emails are saved. UserB is called while her email is looked over. She says she doesn't know how this could have happened. Original emailA is no longer showing in mailbox or in trash and a search can't locate it . In the sent items is a response from userB to userA thanking for document in emailA. There is another sent item which was the emailA being sent to userC and bcc'd to the 8 others. A look at the workinprogress folder reveals emailB in draft form with the To box extended and userC listed.
In my opinion, it looks like userB had multiple windows open, somehow clicked forward on confidential emailA and added the addresses that should have been put on emailB and hit send. Maybe after realizing mistake deleted original emailA and shutdown browser, leaving orphaned composition boxes (hence the recovered emails).

Is there any possible way this could have been caused by a technical glitch? Are there any logs that would be helpful? May have legal issues. She says she " will work with IT to determine how this happened".
Thanks for any help.