Fix: SmtpSever trying to use an unpermitted port (#33)

* Add method trying to bind an available port
* Removed console message for CanBindPort(int port)

Src/MailMergeLib.Tests/Helper.cs

@@ -3,6 +3,7 @@
 using System.Linq;
 using System.Net;
 using System.Net.NetworkInformation;
+using System.Net.Sockets;
 using System.Reflection;
 
 namespace MailMergeLib.Tests;
@@ -51,11 +52,11 @@ internal static int Compare(Stream? a, Stream? b)
     /// <returns>The first free TCP port found.</returns>
     /// <exception cref="InvalidCastException">If no free port could be found.</exception>
 
-    internal static int GetFreeTcpPort(int startPort = 1) 
+    internal static int GetFreeTcpPort(int startPort = 2000) 
     {
-        for (var i = startPort; i <= Char.MaxValue; i++)
+        for (var i = startPort; i <= 0xFFFF; i++)
         {
-            if (IsFreePort(i)) return i;
+            if (IsFreePort(i) && CanBindPort(i)) return i;
         }
 
         throw new InvalidOperationException("No free TCP port found");
@@ -68,4 +69,21 @@ private static bool IsFreePort(int port)
         var openPorts = listeners.Select(item => item.Port).ToArray<int>();
         return openPorts.All(openPort => openPort != port);
     }
+
+    private static bool CanBindPort(int port)
+    {
+        try
+        {
+            var localEndPoint = new IPEndPoint(IPAddress.Any, port);
+            using var listener = new Socket(IPAddress.Any.AddressFamily, SocketType.Stream, ProtocolType.Tcp);
+            listener.Bind(localEndPoint);
+        }
+        catch
+        {
+            // e.g. because of "Permission denied" or other reason
+            return false;
+        }
+
+        return true;
+    }
 }