Transferring lots of messages between IMAP accounts

I have to move many, many emails (over a thousand) from an old IMAP hosting provider to a new IMAP hosting provider. I know I can just set up the old and new accounts in Thunderbird and drag and drop, but it's very slow and keeps timing out. Is there a better, more automated way to copy all messages (and all mailboxes) between IMAP servers?

There are couple of options you could try:

• Check out Google Apps IMAP migration guide, you can setup an Google Apps IMAP account as a temporary placeholder account while you do the transfer.
• Use a third party service like YippieMove. Note that this costs $15 per account
• Do it yourself! A blog post on how you can transfer mails using PHP

IMAPSync is the tool your looking for. IMAPSync

In the FAQ there are plenty of good examples

Use the reliable Mutt (http://www.mutt.org/).

1. mutt -f imap://username@sourceimaphost/INBOX/folder
2. Tag selected messages by t, or tag all messages by T and entering ~A. (With T, you can specify various patterns (http://www.mutt.org/doc/manual/manual-4.html#ss4.2).
3. Move tagged messages by ;s("save") and enter imap://username@destimaphost/INBOX/folder. (This marks the source messages as deleted(D); if you don't want that, use ;C("copy") instead.)
4. You can repeat 2 and 3 as needed.
5. Quit by q. You may choose to purge the deleted messages.

This allows to move a folder from one account to another. Repeat this for multiple folders.