The Internet Mail Service Sample Extension dynamic-link library (Imcext.dll) intercepts inbound messages from simple mail transport protocol (SMTP) hosts and selectively reroutes those messages to other hosts before they are processed by the Internet Mail Service. This enables the Internet Mail Service to act as a smart host that can route messages between the Internet and other SMTP hosts.
For example, if your organization uses multiple SMTP hosts, this extension enables you to designate the Internet Mail Service as the single point of contact to the Internet. Other SMTP hosts can be configured to forward all messages to the Internet Mail Service, where the extension will route messages appropriately.
The sample DLL modifies envelope addresses. However, it doesn't modify any message headers or the message body.