You set up both accounts in Thunderbird using IMAP. Then you can drag and drop messages between accounts. I am personally uneasy about drop-and-dragging folders, though many users do it and it seems to work. One of my reservations is that it's quite easy to hiccup with drag-and-drop and drop your files/folders somewhere you hadn't intended. An alternative method, using a right-click menu offers a "move to" action which is less prone to accidents. Its drawback is that it doesn't offer to move folders. For myself, I would prefer to do the extra work; I would create the folders by hand and then copy their contents, one folder at a time, Ctrl+a lets you quickly select all the messages in a folder. When you move or copy messages between accounts, you are also (in some cases) asking for them to be downloaded from one server, and uploaded to another. Bear this in mind, and that the movement of large amounts of data, or large numbers of messages, will take some time. It would be quite easy to grab and drag a folder with a complicated system of subfolders and in doing so, ask it to move mega or gigabytes of data. Doing it one folder at a time, for instance, and allowing each folder copy to complete before moving on to the next would be rather safer and more likely to work