listen

Explanation
Sieve is a language for filtering and sorting email messages on a mail server. Sieve core filters are the basic actions that can be applied to a message that matches a set of conditions. The following actions are available in Sieve core filters:
discard: This action discards the message silently, without sending any notification to the sender or the recipient. This action is useful for deleting spam or unwanted messages.
fileinto: This action delivers the message to a specified mailbox. The mailbox can be an existing one or a new one that is created by the action. This action is useful for organizing messages into different folders based on their content or headers.
reject: This action rejects the message and sends a notification to the sender with a specified reason. The message is not delivered to the recipient. This action is useful for informing the sender that the message was not accepted due to some policy or error.
The other options are not valid actions in Sieve core filters. drop, relay, and fileinto are not recognized keywords by Sieve.
References:
Sieve: An Email Filtering Language, section 2.10, "Actions"
Sieve Tutorial, section 4, "Actions"