Antix SMTP Impostor (formally SMTP Server for Developers) allows you to configure and test the way SMTP servers work. You will be able to create as many hosts as you want on multiple ports.
Antix SMTP Impostor will not actually send emails, its made for testing purposes only.
Antix SMTP Impostor Features:
1. Fake - will not actually send e-mails, no risk of accidentally e-mailing a client then
2. Testable - you can create the server in code, then test your e-mailing functions and check the server to confirm they have been sent
3. Configurable - you can add as many hosts as you want listening on different ports
4. Expendable - you can write you own message storage, perhaps to store in a database

The license of this software is Free, you can free download and free use this server utility software.