I was installing horde on a new server today and had to migrate my imap folders from zimbra to postfix. Both servers run under ubuntu 12.04 LTS. I was trying some different tools, however the only working solution i found was to use imapsync: https://github.com/patkar/imapsync. To get it running on ubuntu you have to install the following packages:
apt-get install libterm-readkey-perl libmail-imapclient-perl and additionally from cpan:
perl -MCPAN -e shell
Then you can start with something like:
./imapsync –host1 host.de –user1 firstname.lastname@example.org –password1 pw1 –host2 –user2 email@example.com –password2 pw2 –noauthmd5 –ssl1
You have to use noauthmd5 and ssl1 for zimbra to be able to connect to the imap server, without these options it did not work for me.