Hi,
We’ve just transferred over many Gigabytes of email using the free version of imapsync which can be found at https://github.com/imapsync/imapsync. We know there is a non free version but this one is free and works well for us.
This is a Perl script and you will need to download a reasonable number of Perl modules to make it work. If you are used to Perl, you need CPAN. One of the modules Sys::MemInfo will not install through cpanm so we had to do
sudo apt-get -y install libgd2-xpm-dev build-essential
sudo perl -MCPAN -e 'install Sys::CPU'
perl -MCPAN -e 'install Sys::CPU
cpanm Sys::CPU
cpanm Sys::MemInfo
to force it in.
The rest of the modules were fine. The syntax for imapsync is very easy.
We tried mbsync and imapoffline and they were a nightmare to get going, very confusing to us.
imapsync just worked with our LetsEncrypt certificate with zero issues first time.
Best wishes,
Rob