外部送信チェック for Gmail is a Chrome extension designed to enhance email security by prompting users to confirm external recipients and attachments before sending. This tool provides a modal dialog that lists external domain recipients and any attached files, ensuring that users review their email content to prevent accidental data leaks. It effectively blocks the sending of emails until all items are verified, offering a safeguard for both individuals and organizations.

Key features include blocking the send button and keyboard shortcuts, grouping external recipients by domain for easier review, and optional checks for sender information. The extension also supports various Gmail functionalities like thread replies and scheduled sending, while maintaining user privacy by processing all information locally within the browser. With a user-friendly interface available in both Japanese and English, it integrates seamlessly with Gmail's Material Design.