Why is it everyone thinks the IMAP mail protocol is some sort of file system. It is a mail system that works on synchronizing mail between folders. This does provide an opportunity to add mail directly to the server, but it is not a file system. What adding a mail to the imap server folder does mean is a lot of back and forth conversation between the mail server and the mail client. This conversation is a significant over head beyond just the mail data. Remember also that your internet connection speed is typically measured in bits. and upload is throttled at around 50% or less than your download speed so a download speed of 20mbits will in ideal circumstances upload 2.5MB per second. out of that you have to deduct the TCP overhead of about 13% - 20% of that 2.5Mb. Then there is the other things you are doing on that line that it is sharing with like streaming music or video of downloading mail. Everything coming down uses uploads to communicate. Now lets assume the mail server is busy and drops a few packets and in fact simply looses a couple of messages because it is to busy to get the message back to the client because you just hit it with 20,000 new items and they are coming in faster that the server has resources allocated to cope with this unlikely event. Notmaly a sent mail is about what the server administrator has resourced the server to receive from the mail client as new items. perhaps a dozen if you do a mail merge. The bottom line is IMAP is not suited as a file system or a mail backup client and trying to synchronize up stream more than about 500 emails at a time will almost always end in frustration and tears. This in no reflection on Thunderbird or the mail server. It is a reality of using IMAP. Is there a better method. Personally I recommend examining why you are even doing it. Because the best solution is not upload anything you do not need is my opinion. Do you need to add to the knowledge Google or Yahoo or whomever it is has about you by adding all your mail from years ago, just so they can build a better profile of you to market things to you or sell to someone else.