You can enhance security when connecting to the Internet by configuring the following options on Internet Mail Service. For information about how to set these options, see Microsoft Exchange Server Operations.
Reject messages by IP address By default, Internet Mail Service accepts incoming connections from any IP address. If you want Internet Mail Service to communicate only with specific SMTP hosts, you can configure it to reject connection attempts from other IP addresses. This makes it more difficult for someone to mount an attack from the Internet against your system.
Set message size limits You can establish message-size limits for Internet Mail Service. The default limit applies to both incoming and outgoing mail. If an incoming message from another SMTP host exceeds the specified limit, Internet Mail Service stops writing data to disk and discards any remaining data. This prevents large messages from filling up the disk on your server, which helps reduce the impact of a denial-of-service attack.
Disable auto-replies to the Internet Internet Mail Service can prevent the delivery of automatically generated replies, such as out-of-office replies. When users set up out-of-office replies on their clients, they frequently include a number where they can be reached and also other information. In some cases, this information should not be shared outside the organization. You can configure Internet Mail Service to disable outbound delivery of automated replies globally or by domain.
Set delivery restrictions You can restrict what users in your organization have permission to send mail through Internet Mail Service. For example, you can grant Internet mail access to full-time employees. You can grant or deny Internet mail access to users by configuring the Internet Mail Service Delivery Restrictions property page in the Administrator program.