Incoming Mail Accounts - POP3 options

This screen gives you a list of options available for POP3 Mail Accounts.

Keep Messages on Server
Sets whether messages stay on the server after download, or are removed.
Delete Messages on Server On Local Deletion
If Keep Messages on Server is set, then this determines if messages on the server are removed when they are deleted on the local client. This is the normal setting for a single or primary client, but may be useful to unset if you are accessing mail from multiple clients.
Maildir backend
Sets this to use a Maildir (single file per message) formatted backend rather than an mbox (single file per folder) folder. Recommended.