Fix unneeded breaking change with requirement of MailTarget.SmtpServer

src/NLog/Targets/MailTarget.cs

@@ -173,7 +173,6 @@ public Layout Body
         /// Gets or sets SMTP Server to be used for sending.
         /// </summary>
         /// <docgen category='SMTP Options' order='10' />
-        [RequiredParameter]
         public Layout SmtpServer { get; set; }
 
         /// <summary>
@@ -263,6 +262,18 @@ protected override void Write(AsyncLogEventInfo[] logEvents)
             }
         }
 
+        /// <summary>
+        /// Initializes the target. Can be used by inheriting classes
+        /// to initialize logging.
+        /// </summary>
+        protected override void InitializeTarget()
+        {
+
+            CheckRequiredParameters();
+
+            base.InitializeTarget();
+        }
+
         /// <summary>
         /// Create mail and send with SMTP
         /// </summary>
@@ -367,6 +378,9 @@ private StringBuilder CreateBodyBuffer(IEnumerable<AsyncLogEventInfo> events, Lo
         /// <param name="client">client to set properties on</param>
         private void ConfigureMailClient(LogEventInfo lastEvent, ISmtpClient client)
         {
+
+            CheckRequiredParameters();
+
             var renderedSmtpServer = this.SmtpServer.Render(lastEvent);
             if (string.IsNullOrEmpty(renderedSmtpServer))
             {
@@ -392,6 +406,15 @@ private void ConfigureMailClient(LogEventInfo lastEvent, ISmtpClient client)
             }
         }
 
+        private void CheckRequiredParameters()
+        {
+            if (!this.UseSystemNetMailSettings && this.SmtpServer == null)
+            {
+                throw new NLogConfigurationException(
+                    string.Format("The MailTarget's '{0}' property is not set - but needed because useSystemNetMailSettings=false. The email message will not be sent.", "SmtpServer"));
+            }
+        }
+
         /// <summary>
         /// Create key for grouping. Needed for multiple events in one mailmessage
         /// </summary>

tests/NLog.UnitTests/Targets/MailTargetTests.cs

@@ -635,15 +635,31 @@ public void MailTargetInitialize_WithoutSpecifiedFrom_ThrowsConfigException()
         }
 
         [Fact]
-        public void MailTargetInitialize_WithoutSpecifiedSmtpServer_ThrowsConfigException()
+        public void MailTargetInitialize_WithoutSpecifiedSmtpServer_should_not_ThrowsConfigException()
         {
             var mmt = new MockMailTarget
             {
                 From = "foo@bar.com",
                 To = "bar@bar.com",
                 Subject = "Hello from NLog",
                 SmtpPort = 27,
-                Body = "${level} ${logger} ${message}"
+                Body = "${level} ${logger} ${message}",
+                UseSystemNetMailSettings = true
+            };
+
+        }
+
+        [Fact]
+        public void MailTargetInitialize_WithoutSpecifiedSmtpServer_ThrowsConfigException_if_UseSystemNetMailSettings()
+        {
+            var mmt = new MockMailTarget
+            {
+                From = "foo@bar.com",
+                To = "bar@bar.com",
+                Subject = "Hello from NLog",
+                SmtpPort = 27,
+                Body = "${level} ${logger} ${message}",
+                UseSystemNetMailSettings = false
             };
             Assert.Throws<NLogConfigurationException>(() => mmt.Initialize(null));
         }