How do I copy a large number of emails to an IMAP folder in Thunderbird?

I am migrating all my email to a new server. I have about 10 accounts in 10 different folders in Mozilla Thunderbird. I have been using pop3 in the past, but on the new server I want to use imap.

I have created all the imap accounts on the new server and I want to copy over all the emails on their folders on Thunderbird to their corresponding imap account folders on the new server.

When searching for an answer for this, most people suggest just copying all the emails in their folder to the imap account inbox. Some people say that you should only copy around 500 emails at a time. The problem is I have around 20,000 emails per account (going back to the year 2002!) and Thunderbird just gives up on me. I have managed to get it working occassionally by copying 500 emails at a time but this is going to take forever!

Thunderbird shows that it is copying the emails with a message in the status bar, but after a short while the status message goes blank.

My question is, is there a way of moving a large number of emails from a local folder on Thunderbird to an imap account that works and is quick?

I have used Thunderbird to move large quantities of messages from Local to imap. You'll need to ensure that your local folders are indexed properly, and then start with smallish chunks and keep adding more until you get to a comfortable number. The last time I did this I started with about 1000, then 2500, then 5000, and pretty soon I was done. It could take some time, but be patient and try to leave thunderbird alone to do it's work while moving.

Get the add-on named Copy Folder. Then go to Tools > Add Ons, then search "copy folder" and follow prompts. After restarting Thunderbird you simply right-click the folder and it will now give the option of Copy.