I'm a happy user of Thunderbird Portable 38.3.0. I have configured to store my replies to the same folder where the message I reply to is. When replying to messages in my inbox *and* having (rare) network problems with the service provider, sometimes I end up loosing my reply: the outgoing server worked, so the message was sent, but then "Saving the message to Inbox" just tries and tries and does not succeed. Here I have the Thunderbird dialog options to retry and cancel, but not an option to store into another (e.g. local) folder. Of course, Thunderbird could automatically in such case when saving takes longer than usual, store a copy in the local Sent-folder. This would be essential, as loosing my copy of important replies is really not acceptable.