I really want a tool that could somehow create offline copies of my emails. I know, there are tools like Mail Steward, but for the same, I need to find some time separately, sit and archive the emails manually. My system doesn't sleep for like 7-8 days at a stretch. An automated fashion could be more helpful, I guess. Any recommendation for the same? I use an iMac 2016 model upgraded to 10.13.6
Mail Steward is known to offer manual backup only. for an automated way, you may give a try to Mail Backup X. As far as I know, this is the only application that offers automatic backup. it is supported well on your OS version. for a quicker search: Backup mails automatically to your system.