The easiest way I discovered for importing loads of Thunderbird regional folders should be to setup a temporary dovecot server, change the thunderbird mail suppliers to linux mbox mailstores and then imapsync it from the temp server into the zimbra server: It is suggested to copy the export data files https://free-webmail-zimbra-migra90000.illawiki.com/728940/not_known_factual_statements_about_free_mail_zimbra_migration