If sending the messages you obtain the error: Server error 553, Error number 0x800ccc79

Impossible to send the message. One of the recipients has not been accepted by the server.
The e-mail address not accepted by the server is
xxxxxx@xxxxx.it. 'Prova' subject, Account test, Server:'smtp.domainname.xxx, SMTP Protocol, Server answer:'553 sorry, that domain isn't in my list of allowed rcpthosts (#5.7.1)', Door: 25, Protection (SSL): No, Server Error 553, Error number 0x800ccc79

The error "553 sorry, that domain isn't in my list of allowed rcpthosts (#5.7.1)" shows that you are trying to send mail through an SMTP inconsistent with the connection. In fact the Simple Mail Transfer Protocol must be relevant exclusively to the Provider which you use to connect yourself.
Therefore you can use 'smtp.domainname.xxx' only if connected with Aruba.it or if the Antivirus & Antispam service has been activated: otherwise you need to contact your ISP connection.