The only time I've ever lost mail was while trying to move a bunch of messages in Postbox (shares Thunderbird's backend). Wasn't even moving between servers--

The only time I've ever lost mail was while trying to move a bunch of messages in Postbox (shares Thunderbird's backend). Wasn't even moving between servers-- this was just a folder-to-folder move. Always copy, verify that the messages really are at the destination, then delete. I don't know why this isn't what Thunderbird does on its own when you move mail, but it isn't.