Email deleted

how do I keep mail in client after deleting from server? Do I have to archive every one I want to keep?

That depends on how you are receiving your email from the server.

If your account is setup as POP3, once the email is downloaded you can delete it from the server and it will remain in eM Client.

If your account is setup as IMAP, eM Client is simply a synced copy of what is on the server. So if you delete something on the server, it will delete it from eM Client. Or if you delete something in eM Client, it will delete it from the server. If you want to delete messages from the server, but keep them in eM Client, then you will either have to copy them to Local Folders, or as you suggested, Archive them.