Adding a new domain

I signed for a 10 user trial account.

When I go to add my domain, I get this alert after creating it:

I added a domain with this info:

WARNING: Routes check is failed for given domain.
Below is detailed response from mail server :
Route check failed, code: 550, error: No Such User Here"

I don’t understand what user is referring, and what is the problem in detail.
Please anyone can help me?

Hello magostoni,

You might want to check this explanation I received when I posted the same question few months ago: