keep;
The "keep" action is whatever action is taken in lieu of all other actions, if no filtering happens at all; generally, this simply means to file the message into the user's main mailbox. This command provides a way to execute this action without needing to know the name of the user's main mailbox, providing a way to call it without needing to understand the user's setup, or the underlying mail system.
For instance, in an implementation where the IMAP server is running scripts on behalf of the user at time of delivery, a keep command is equivalent to a fileinto "INBOX".
if size :under 1M { keep; } else { discard; }
Note that the above script is identical to the one below.
if not size :under 1M { discard; }
Core : Requires no import statement
RFC3028