I see no harm doing a MX lookup with checkdnsrr() and I also don’t see how false positives may appear. You don’t need to escape the hostname, in fact you can use this technique and take it a little further by talking to the MTA and testing if the user exists at a given host … Read more
Update 2018/5/23: Check out MichaC’s answer for a newer library that has .NET standard support. Original Answer: The ARSoft.Tools.Net library by Alexander Reinert seems to do the job pretty well. It’s available from NuGet: PM> Install-Package ARSoft.Tools.Net Import the namespace: using ARSoft.Tools.Net.Dns; Then making a synchronous lookup is as simple as: var resolver = new … Read more