diff --git a/FlexiMail.Tests.Unit/Services/FlexiExchangeServiceTests.Validations.cs b/FlexiMail.Tests.Unit/Services/FlexiExchangeServiceTests.Validations.cs
new file mode 100644
index 0000000..5cee250
--- /dev/null
+++ b/FlexiMail.Tests.Unit/Services/FlexiExchangeServiceTests.Validations.cs
@@ -0,0 +1,56 @@
+// ---------------------------------------
+// Copyright (c) 2024 Mabrouk Mahdhi.
+// Made with love for the .NET Community
+// ---------------------------------------
+
+using System;
+using FlexiMail.Models.Foundations.Messages;
+using FlexiMail.Models.Foundations.Messages.Exceptions;
+using FluentAssertions;
+using Microsoft.Exchange.WebServices.Data;
+using Moq;
+
+namespace FlexiMail.Tests.Unit.Services
+{
+    public partial class FlexiExchangeServiceTests
+    {
+        [Fact]
+        public void ShouldThrowValidationExceptionIfFlexiMessageIsNull()
+        {
+            // given 
+            FlexiMessage nullMessage = null;
+
+            var nullFlexiMessageException =
+                new NullFlexiMessageException(
+                    message: "FlexiMessage is null.");
+
+            var expectedFlexiMessageValidationException =
+                new FlexiMessageValidationException(
+                    message: "Flexi Message validation error occurred, fix errors and try again.",
+                    innerException: nullFlexiMessageException);
+
+            // when
+            void SendMessageAction() => this.flexiExchangeService.SendAndSaveCopyAsync(nullMessage);
+
+            var actualException = Assert.Throws<FlexiMessageValidationException>((Action)SendMessageAction);
+            // then
+
+            actualException.Should().BeEquivalentTo(expectedFlexiMessageValidationException);
+
+            this.exchangeBrokerMock.Verify(broker =>
+                    broker.GetAccessTokenAsync(),
+                Times.Never);
+
+            this.exchangeBrokerMock.Verify(broker =>
+                    broker.CreateExchangeService(
+                        ExchangeVersion.Exchange2013,
+                        It.IsAny<string>(),
+                        It.IsAny<ImpersonatedUserId>()),
+                Times.Never);
+
+            this.exchangeBrokerMock.Verify(broker =>
+                    broker.SendAndSaveCopy(It.IsAny<EmailMessage>()),
+                Times.Never);
+        }
+    }
+}
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diff --git a/FlexiMail/Models/Foundations/Messages/Exceptions/FlexiMessageValidationException.cs b/FlexiMail/Models/Foundations/Messages/Exceptions/FlexiMessageValidationException.cs
new file mode 100644
index 0000000..f3f2fcd
--- /dev/null
+++ b/FlexiMail/Models/Foundations/Messages/Exceptions/FlexiMessageValidationException.cs
@@ -0,0 +1,14 @@
+// ---------------------------------------
+// Copyright (c) 2024 Mabrouk Mahdhi.
+// Made with love for the .NET Community
+// ---------------------------------------
+
+using System;
+
+namespace FlexiMail.Models.Foundations.Messages.Exceptions
+{
+    public class FlexiMessageValidationException(string message, Exception innerException)
+        : Exception(message, innerException)
+    {
+    }
+}
\ No newline at end of file
diff --git a/FlexiMail/Models/Foundations/Messages/Exceptions/NullFlexiMessageException.cs b/FlexiMail/Models/Foundations/Messages/Exceptions/NullFlexiMessageException.cs
new file mode 100644
index 0000000..9a8c3b0
--- /dev/null
+++ b/FlexiMail/Models/Foundations/Messages/Exceptions/NullFlexiMessageException.cs
@@ -0,0 +1,13 @@
+// ---------------------------------------
+// Copyright (c) 2024 Mabrouk Mahdhi.
+// Made with love for the .NET Community
+// ---------------------------------------
+
+using System;
+
+namespace FlexiMail.Models.Foundations.Messages.Exceptions
+{
+    public class NullFlexiMessageException(string message) : Exception(message)
+    {
+    }
+}
\ No newline at end of file