Simple Mail Transfer Protocol is a service provided by IIS for delivering outgoing e-mail messages. In case you will get an SMTP error 550 Unable to relay it means your server is not allowing relaying.

To workaround this issue you will need to allow relaying for your ip address:

1) Open IIS Manager

2) Right Click Local Server and click Properties

3) On the Access Tab and Under Relay Restrictions click on Relay

4) Grant Access to 127.0.0.1 ip address.

5) Close all dialogs and restart SMTP.