see below from an answer of John Galis:
“backup should be located in Documents/eM Client folder, you can setup different folder in Tools - Settings - General - Backup”
exactly there 
Thank you for helping me.
you’re welcome
Hi,
There are more options to preserve our emails and our backups. For example Mailstore is a good choice, although there are other options like OpenKM and emails module, which allows us to use the repository as a document management system to store messages massively.
For more info: http://www.openkm.com/en/modules-eng/…
Greetings,
Fuensanta
Hi, there is no need to use external software, eM Client can do backups by itself.
Jan
I feel like I am late to this dance. I moved a large number of messages into eM Client from Thunderbird. I have created a backup. It is more than 9.6 gb in size and takes a verrrrrry long time to perform and restore. I use the back up to move mail between my desktop and laptop. If I move messages into archive, will that eliminate them from backups and reduce the size of the back up zip file? When I move messages into archive, how to I regain access to them once archived?
BTW, the documentation on all this in the help manual SUCKS big time! It is useless.
Hello Michael, the Archive folder should be displayed in the left pane once you import some messages into the folder using automatic archiving.
Archive folders are not excluded of the backup, the built-in backup feature back ups all your application data and content.