According to the tails devs a couple of days ago ... "We discovered that Claws Mail, the email client in Tails, stores plaintext copies of all emails on the remote IMAP server, including those that are meant to be encrypted." What the tails guys didn't mention on their site and I think is equally as concerning as the bug itself is that issue was reported to the claws mail bugzilla in 2012, bug #2661, and again in 2013, bug #2965.