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.
Stopbutton to stop the service
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.