From KB:
On initial synchronization, 500 messages are retrieved from the Inbox.
A new menu option “Get more emails” is available in Preferences.
When selected, 500 more emails are retrieved from the folder shown under the option (for example “Current folder: Inbox”) each time the button is pressed.
When the option is repeatedly used and the mailbox reaches 80%, the menu option is unavailable (‘Get more emails’ option is grayed out) and the label says “At maximum email capacity”.
This option can’t be used, but email synchronization for new items will continue.