This error is a bit trickier to resolve only because there can be many different causes. No data connection – test your SMTP connection using telnet. If you are unsure how to do this, there is a good program: Exchange Server Fix Toolbox https://www.fixtoolbox.com/exchangeserverfix.html
Microsoft Exchange Server is unavailable; if you are unable to solve this error then I will tell you no stress and immediately apply to follow steps like