So what’s your question? Perhaps you could rephrase your question?
My eM Client is only a mirror of my mailbox on the web interface. If you delete a message on the web interface (because my inbox is full), email message is deleted also eM Client. I need a message be available to one, two, three years …This is my problem
you can enable Local folders (Tools > Settings > General > Show Local folders). These will look like another account in the left side menu of the application, under your other accounts. If you drag your old emails into these, they will be removed from your server but stay available in your computer. If you do not need to access these you can export them through the File > Export function as eml files and save them anywhere in your computer.
Make sure to backup your database periodically (manually or automatic, also through Settings) to not lose these emails in case something happens with your database or device.
Simplicity is beauty … :o)