If sending an email you get the following error message:

Unable to send the message. One of the recipients was rejected by the server.
The e-mail address is not accepted This e-mail address is being protected from spambots. You need JavaScript enabled to view it . Subject 'Message Subject', Account: ' This e-mail address is being protected from spambots. You need JavaScript enabled to view it ,' Server ' This e-mail address is being protected from spambots. You need JavaScript enabled to view it ', Protocol: SMTP, Server Response: '550 5.7.1 <...> Relaying denied.
Proper authentication rest. 'Port: 25, Secure (SSL): No, Server Error 550, Error Number: 0x800CCC79

Most likely you need to set the outgoing server authentication, following the setting for Outlook Express:
go to the Tools menu - Accounts - Properties - Server and tick the box 'My server requires authentication'