When trying to send emails from your email client software you get an error in your email client 550 5.7.1 Unable to relay for …

This error is happening because you have installed the Microsoft IIS SMTP Service, and your email client is talking to that service rather than to VPOP3.

To resolve this, you need to close down the Microsoft SMTP service and restart VPOP3.

1. Go to Start → Control Panel → Administrative Tools → Services in Windows.
2. Find the Simple Mail Transfer Protocol service (sometimes called the Microsoft Simple Mail Transfer Protocol or Microsoft SMTP Service etc)
3. Double-click on this service to get the settings
4. Press the Stop button to stop the service
5. Set the Startup Type to Disabled.
6. Find the VPOP3 Email Server service
7. Right-click it and choose Restart

NB - In IIS 7, the Microsoft SMTP service is called Simple Mail Transfer Protocol (SMTP)

We haven't tried this here, but a user with SBS 2011 said he fixed this problem as below:

 The cause of the problem was SBS 2011. This comes with Exchange, 
 and I had partially set this up before giving it up as too intensive. 
 The ESMTP service was running as part of the client hub for 
 Exchange, so disabling the hub services within Exchange Manager 
 Console fixed the problem.