If it was easy, you were lucky. Seriously, or only had a very small amount of mail. Between the fact IMAP is not a file system and therefore not robust when more than a few hundred mail are involved and Thunderbird's inherent bugs (some have been fixed in the past year or so, but not all timeout errors are issued instead of simply being ignored for instance) invariably some data is lost. I would not recommend it as an approach for anything of any value. You can ensure you have no anti virus and nothing else interfering in network connectivity to maximize your chances of having a dream run, and many folks have written blog post on the internet about how simple it is. But it is still pot luck. Add that to folk like Gmail which actually have a daily limit on bandwidth they offer for free and you start to get a picture. Find another solution is how I read it. Most business hosting services will transfer you mail from the old to new server for a fee. The free approach is not really something to consider with business records. But if you are insistent on pursuing the approach. I would be using Ctrl+A for select all. Ctrl+C for copy, or using drag and drop. I certainly would try and avoid move as if it fails, the mail has left the existing folder and never arrives in the new destination.