In JetBackup 4, restoring emails errors may occur if a user tries to download mails at the time of the restore or if a user received an email during the restore process as this will create mail folders.
As a workaround, you may download the email folder {domain/email} from the backup and manually sync the files to their location. If you download the folder from the cPanel interface, the download will be stored in /home/user/.jbm.
If you download the folder from the WHM interface, the download will be stored in /usr/local/jetapps/usr/jetbackup/downloads.