The SMTP-proxy policy can be configured to restrict email messages and email content based on the following characteristics: the sender's Mail From address, the size of the email message, the attachment file name and content type, and the maximum number of email recipients. These characteristics can help manage and secure email traffic. The sender’s Mail From address can be used to filter emails from specific domains. The email message size can be limited to prevent excessively large attachments. The attachment file name and content type can be controlled to block potentially harmful files. Lastly, setting a maximum number of email recipients can help prevent spam emails from being sent to a large number of addresses. Using WebBlocker to check URLs in messages is not applicable as WebBlocker is designed for HTTP traffic, not SMTP.