I have installed IIS on my local machine. The SMTP is setup to then relay the messages through a remote smtp server. If I just drop email text files in the pickup directory it works fine. However, in my application, creating the same email message returns the following error:
The server rejected one or more recipient addresses. The server response was: 550 5.7.1 Unable to relay for email@example.com
(obviously I've blanked out the email address)
The same code works fine if I set it up to send it to the remote server directly but we don't want to do this.