The correct option is Mail retriever.

The Mail retriever is the component that connects to the linked inbox and polls for unread messages. It performs the periodic retrieval that copies messages into the mail database so that subsequent stages can apply filters for auto replies and spam and the cleanup job can enforce retention.

The Email processor is not correct because that term refers to processing logic that runs after messages are already in the system rather than the component that polls an external inbox.

The Cloud Pub/Sub option is not correct because Pub/Sub is a messaging service used to deliver events inside cloud architectures and it does not itself poll or retrieve email from a mailbox.

The Mail handler is not correct because a handler typically operates on messages once they have been retrieved and stored rather than performing the external retrieval from the connected inbox.

The Inbound mail gateway is not correct because a gateway normally accepts incoming SMTP deliveries or routes mail into the system and does not match the described polling retriever that fetches unread messages from a remote mailbox.

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