If you want to change your webmailer from Roundcube to RainLoop, you only have to backup and import the saved addresses from your mailbox once.
Export contacts
To export your contacts, log in to Roundcube, select “Contacts” on the left and then click on “Export”.
Import contacts
To import the contacts, log in to RainLoop, go to Contacts on the left and select “Import (csv, vcf, vCard)” and then upload the exported contacts.