Time Machine has served Mac users for years. However, there are complaints about TM during retrieval procedure. TM can backup the folders easily, but while retrieving either the emails aren’t found, or the restoration process is complicated. As an alternative, you can export the mails to mbox format manually on a fixed interval (like two months or so). You can save this file at the desired location away from the system hard disk. You can import this file to your Postbox later, when in need. The last option will be to get a paid backup tool that could back up and restore Postbox easily. I use Mail Backup X for the same. It creates mail backup of my Apple Mail. Auto Archiving option along with incremental backup ensure that no manual intervention is needed in the process. Take the free trial of the application before purchasing. Though it is easy to configure, if you still feel any issue while creating mail backup, you can contact the support team for free.
If you are on IMAP server, you won’t need much of backup as a copy always stays at the server and can be retrieved at the other system easily. However, there are instances when users have lost emails for no reason. Something that shocks more is- the provider doesn’t respond them well. However, that is a one in a million case. If you are on a POP server, you should get a backup. Dependency on the system performance might end you up losing all the emails. Thunderbird keeps its mail folder hidden for security reasons. Exporting the emails to MBOX format manually can save your emails. Copy the backed up data to an external location to avoid issues due to system errors. Mail Backup X is a potential alternative for the same. It backs up mails automatically to the desired location. You can store the offline copy to local storage or any online storage space. It offers a 15-day free trial before the purchase.