If you are not interested in the technical details, but just want to migrate your LD E-Mails, check out E-Mail Migration - LD Mail (in German).
When switching from one webspace to another or from one email provider to another, one of course wants to move existing data too. Obviously, some of them will be critical for your own business. There are several methods to do so. All of them require that the email accounts in both mailservers are still up and running.
This is most likely the easiest and safest methos for end-users. However, it requires that IMAP Sync is configured as described in the linked page. Imap Sync will have to be triggered by using HTML & PHP afterwards. A proof of concept on how to achieve that may be found on GitHub under liberaledemokraten/php-email-migration.
The manual approach is probably among the easiest to go as everyone would just register their accounts on both servers using the same email client (e.g. Thunderbird has the easiest approach here). Thenafter, he may just drag & drop copy or move all the emails from the old account to the new one. Each folder's content to the respective folder in the new account.
To do so, an application must be used that gets feeded in the login credentials to both the old and new account and copy all emails from one account to the other. For instance, imapsync is one such application. More information can be found within this documentation under the sub-page IMAP Sync.
This method seems to not work. The plugin is probably outdated and not working with newer roundcube versions. Thus this is still displayed here just to keep the method archived.
This is the easiest and a self-managed method. It is the recommended method as it can be done through our webmail interface without requiring any troublesome manual work. More details on this can be found in GitHub server-gurus/RCimapSync.
The usage is pretty easy. You just have to enter the IMAP credentials of the e-mail account with your previous provider and let the Roundcube plugib deal with downloading the messages into the mailbox that you are currently logged in to.
Essentially, it is also possible to run both accounts in parallel. In the future, it'd also be possible to set up a secondary mailserver on another location and snchronize the accounts on both server with each other. That way, we could improve the reachability in case of a downtime. In combination with a loadbalancer, we could even run both mailservers under the same domain name.
cd /path/to/your/roundcube/plugins/ git clone https://github.com/server-gurus/RCimapSync.git ./imapsync
$config['plugins'] = array('plugin1','plugin2',[...],'imapsync');
mysql.initial.sqlto your mysql database
viewfor the table
imapsyncif your imported table name is different
/bin/directory outside the webroot
/bin/folder and rename
config.confand add the credential information to your database
libunix-syslog-perl) on Debian-based systems