(moved from dotnet/core#2031)

The old implementation of SmtpClient (https://referencesource.microsoft.com/#system/net/System/Net/mail/SmtpClient.cs) was able to use the clientDomain from configuration MailConfiguration.Smtp.Network.ClientDomain;

  <smtp>
     <network host="somehost" port="25" clientDomain="mymachine.mydomain.com" enableSsl="false" />
  </smtp>

solving problems like this one: https://stackoverflow.com/questions/37410598/fqdn-with-smtpclient-in-powershell

But in the .Net Core implementation https://github.com/dotnet/corefx/blob/master/src/System.Net.Mail/src/System/Net/Mail/SmtpClient.cs only the fallback to IPGlobalProperties.GetIPGlobalProperties().HostName; is implemented.

Unfortunately, I get a SmtpException because my Smtp server requires fully-qualified hostnames:

Maybe we can find a solution by changing the SMTP server, but our admins say that FQDN check is a POSTFIX default setting, so I'm surprised I'm the only one having this problem.

Thanks