diff --git a/Mindscape.Raygun4Net.Core/IRaygunMessageInitializer.cs b/Mindscape.Raygun4Net.Core/IRaygunMessageInitializer.cs
new file mode 100644
index 00000000..793df0e0
--- /dev/null
+++ b/Mindscape.Raygun4Net.Core/IRaygunMessageInitializer.cs
@@ -0,0 +1,20 @@
+using Mindscape.Raygun4Net.Messages;
+
+namespace Mindscape.Raygun4Net
+{
+  /// <summary>
+  /// Represents an initializer for <see cref="RaygunMessage"/> objects.
+  /// </summary>
+  /// <remarks>
+  /// The Raygun Clients use registered <see cref="IRaygunMessageInitializer"/>
+  /// to automatically add additional properties to <see cref="RaygunMessage"/> objects.
+  /// </remarks>
+  public interface IRaygunMessageInitializer
+  {
+    /// <summary>
+    /// Initializes properties of the specified <see cref="RaygunMessage"/> object.
+    /// </summary>
+    /// <param name="raygunMessage">The <see cref="RaygunMessage"/> to initialize.</param>
+    void Initialize(RaygunMessage raygunMessage);
+  }
+}
\ No newline at end of file
diff --git a/Mindscape.Raygun4Net.Core/Mindscape.Raygun4Net.Core.csproj b/Mindscape.Raygun4Net.Core/Mindscape.Raygun4Net.Core.csproj
index 5b219fcb..5099311b 100644
--- a/Mindscape.Raygun4Net.Core/Mindscape.Raygun4Net.Core.csproj
+++ b/Mindscape.Raygun4Net.Core/Mindscape.Raygun4Net.Core.csproj
@@ -121,6 +121,7 @@
       <Link>SimpleJson.cs</Link>
     </Compile>
     <Compile Include="Builders\RaygunErrorMessageBuilder.cs" />
+    <Compile Include="IRaygunMessageInitializer.cs" />
     <Compile Include="Messages\RaygunErrorMessage.cs" />
     <Compile Include="Properties\AssemblyInfo.cs" />
   </ItemGroup>
diff --git a/Mindscape.Raygun4Net.WebApi.Tests/Mindscape.Raygun4Net.WebApi.Tests.csproj b/Mindscape.Raygun4Net.WebApi.Tests/Mindscape.Raygun4Net.WebApi.Tests.csproj
index d3bb63fc..39828cb6 100644
--- a/Mindscape.Raygun4Net.WebApi.Tests/Mindscape.Raygun4Net.WebApi.Tests.csproj
+++ b/Mindscape.Raygun4Net.WebApi.Tests/Mindscape.Raygun4Net.WebApi.Tests.csproj
@@ -49,6 +49,7 @@
   </ItemGroup>
   <ItemGroup>
     <Compile Include="Model\FakeRaygunWebApiClient.cs" />
+    <Compile Include="Model\TestRaygunMessageInitializer.cs" />
     <Compile Include="Model\WrapperException.cs" />
     <Compile Include="Properties\AssemblyInfo.cs" />
     <Compile Include="RaygunWebApiClientTests.cs" />
diff --git a/Mindscape.Raygun4Net.WebApi.Tests/Model/FakeRaygunWebApiClient.cs b/Mindscape.Raygun4Net.WebApi.Tests/Model/FakeRaygunWebApiClient.cs
index 42b0894e..a6cdf491 100644
--- a/Mindscape.Raygun4Net.WebApi.Tests/Model/FakeRaygunWebApiClient.cs
+++ b/Mindscape.Raygun4Net.WebApi.Tests/Model/FakeRaygunWebApiClient.cs
@@ -1,5 +1,6 @@
 using Mindscape.Raygun4Net.Messages;
 using System;
+using System.Collections;
 using System.Collections.Generic;
 
 namespace Mindscape.Raygun4Net.WebApi.Tests.Model
@@ -15,5 +16,10 @@ public IEnumerable<Exception> ExposeStripWrapperExceptions(Exception exception)
     {
       return base.StripWrapperExceptions(exception);
     }
+
+    public RaygunMessage ExposeBuildMessage(Exception exception, IList<string> tags, IDictionary userCustomData, DateTime? currentTime = null)
+    {
+      return base.BuildMessage(exception, tags, userCustomData, currentTime);
+    }
   }
 }
diff --git a/Mindscape.Raygun4Net.WebApi.Tests/Model/TestRaygunMessageInitializer.cs b/Mindscape.Raygun4Net.WebApi.Tests/Model/TestRaygunMessageInitializer.cs
new file mode 100644
index 00000000..1c677aa5
--- /dev/null
+++ b/Mindscape.Raygun4Net.WebApi.Tests/Model/TestRaygunMessageInitializer.cs
@@ -0,0 +1,17 @@
+using System.Collections.Generic;
+using Mindscape.Raygun4Net.Messages;
+
+namespace Mindscape.Raygun4Net.WebApi.Tests.Model
+{
+  public class TestRaygunMessageInitializer : IRaygunMessageInitializer
+  {
+    public void Initialize(RaygunMessage raygunMessage)
+    {
+      if (raygunMessage.Details.UserCustomData == null)
+      {
+        raygunMessage.Details.UserCustomData = new Dictionary<string, object>();
+      }
+      raygunMessage.Details.UserCustomData["initializerTestData"] = "true";
+    }
+  }
+}
\ No newline at end of file
diff --git a/Mindscape.Raygun4Net.WebApi.Tests/RaygunWebApiClientTests.cs b/Mindscape.Raygun4Net.WebApi.Tests/RaygunWebApiClientTests.cs
index 353daa69..f0daffe1 100644
--- a/Mindscape.Raygun4Net.WebApi.Tests/RaygunWebApiClientTests.cs
+++ b/Mindscape.Raygun4Net.WebApi.Tests/RaygunWebApiClientTests.cs
@@ -265,5 +265,14 @@ public void StripReflectionTypeLoadException()
       Assert.IsTrue(ex1.Data["Type"].ToString().Contains("FakeRaygunWebApiClient"));
       Assert.IsTrue(ex2.Data["Type"].ToString().Contains("WrapperException"));
     }
+
+    [Test]
+    public void CanUseInitializerToSetData()
+    {
+      _client.AddMessageInitializer(new TestRaygunMessageInitializer());
+      var message = _client.ExposeBuildMessage(_exception, null, null);
+
+      Assert.IsTrue(message.Details.UserCustomData.Contains("initializerTestData"));
+    }
   }
 }
diff --git a/Mindscape.Raygun4Net.WebApi/RaygunWebApiClient.cs b/Mindscape.Raygun4Net.WebApi/RaygunWebApiClient.cs
index 4d9362cf..99968812 100644
--- a/Mindscape.Raygun4Net.WebApi/RaygunWebApiClient.cs
+++ b/Mindscape.Raygun4Net.WebApi/RaygunWebApiClient.cs
@@ -22,7 +22,6 @@ public class RaygunWebApiClient : RaygunClientBase
     private readonly List<Type> _wrapperExceptions = new List<Type>();
 
     private readonly ThreadLocal<HttpRequestMessage> _currentWebRequest = new ThreadLocal<HttpRequestMessage>(() => null);
-    private readonly ThreadLocal<RaygunRequestMessage> _currentRequestMessage = new ThreadLocal<RaygunRequestMessage>(() => null);
 
     private static RaygunWebApiExceptionFilter _exceptionFilter;
     private static RaygunWebApiActionFilter _actionFilter;
@@ -354,8 +353,6 @@ public void Send(Exception exception, IList<string> tags, IDictionary userCustom
     {
       if (CanSend(exception))
       {
-        _currentRequestMessage.Value = BuildRequestMessage();
-
         StripAndSend(exception, tags, userCustomData);
         FlagAsSent(exception);
       }
@@ -390,15 +387,9 @@ public void SendInBackground(Exception exception, IList<string> tags, IDictionar
     {
       if (CanSend(exception))
       {
-        // We need to process the HttpRequestMessage on the current thread,
-        // otherwise it will be disposed while we are using it on the other thread.
-        RaygunRequestMessage currentRequestMessage = BuildRequestMessage();
         DateTime currentTime = DateTime.UtcNow;
-
-        ThreadPool.QueueUserWorkItem(c => {
-          _currentRequestMessage.Value = currentRequestMessage;
-          StripAndSend(exception, tags, userCustomData, currentTime);
-        });
+        
+        StripAndSendInBackground(exception, tags, userCustomData, currentTime);
         FlagAsSent(exception);
       }
     }
@@ -439,7 +430,7 @@ protected RaygunMessage BuildMessage(Exception exception, IList<string> tags, ID
     protected RaygunMessage BuildMessage(Exception exception, IList<string> tags, IDictionary userCustomData, DateTime? currentTime = null)
     {
       var message = RaygunWebApiMessageBuilder.New
-        .SetHttpDetails(_currentRequestMessage.Value)
+        .SetHttpDetails(BuildRequestMessage())
         .SetTimeStamp(currentTime)
         .SetEnvironmentDetails()
         .SetMachineName(Environment.MachineName)
@@ -455,6 +446,25 @@ protected RaygunMessage BuildMessage(Exception exception, IList<string> tags, ID
       {
         message.Details.GroupingKey = customGroupingKey;
       }
+
+      var initializers = GetRaygunMessageInitializers();
+      foreach (var initializer in initializers)
+      {
+        try
+        {
+          initializer.Initialize(message);
+        }
+        catch (Exception ex)
+        {
+          System.Diagnostics.Trace.WriteLine(string.Format("Error Initializing RaygunMessage {0}", ex.Message));
+
+          if (RaygunSettings.Settings.ThrowOnError)
+          {
+            throw;
+          }
+        }
+      }
+
       return message;
     }
 
@@ -466,6 +476,21 @@ private void StripAndSend(Exception exception, IList<string> tags, IDictionary u
       }
     }
 
+    private void StripAndSendInBackground(Exception exception, IList<string> tags, IDictionary userCustomData, DateTime? currentTime = null)
+    {
+      var messages = new List<RaygunMessage>();
+      foreach (Exception e in StripWrapperExceptions(exception))
+      {
+        messages.Add(BuildMessage(e, tags, userCustomData, currentTime));
+      }
+      // Could use SendInBackground here, but like this we queue multiple sends on a single thread
+      ThreadPool.QueueUserWorkItem(c => {
+        foreach (var message in messages)   {
+          Send(message);
+        }
+      });
+    }
+
     protected IEnumerable<Exception> StripWrapperExceptions(Exception exception)
     {
       if (exception != null && _wrapperExceptions.Any(wrapperException => exception.GetType() == wrapperException && (exception.InnerException != null || exception is ReflectionTypeLoadException)))
diff --git a/Mindscape.Raygun4Net/RaygunClientBase.cs b/Mindscape.Raygun4Net/RaygunClientBase.cs
index 2418c44e..9c792efd 100644
--- a/Mindscape.Raygun4Net/RaygunClientBase.cs
+++ b/Mindscape.Raygun4Net/RaygunClientBase.cs
@@ -1,4 +1,5 @@
 using System;
+using System.Collections.Generic;
 using System.Net;
 using Mindscape.Raygun4Net.Messages;
 
@@ -115,6 +116,44 @@ protected string OnCustomGroupingKey(Exception exception, RaygunMessage message)
       return result;
     }
 
+    /// <summary>
+    /// List of registered <see cref="IRaygunMessageInitializer"/> objects.
+    /// </summary>
+    private List<IRaygunMessageInitializer> RaygunMessageInitializers { get; set; } = new List<IRaygunMessageInitializer>();
+
+    /// <summary>
+    /// Return a copy of the registered <see cref="IRaygunMessageInitializer"/> objects.
+    /// </summary>
+    /// <remarks>
+    /// Ensures that changes made to the collection during processing does not cause problems.
+    /// </remarks>
+    protected virtual List<IRaygunMessageInitializer> GetRaygunMessageInitializers()
+    {
+      return new List<IRaygunMessageInitializer>(RaygunMessageInitializers);
+    }
+
+    /// <summary>
+    /// Register a <see cref="IRaygunMessageInitializer"/> to initialize properties on the <see cref="RaygunMessage"/>.
+    /// </summary>
+    /// <param name="raygunMessageInitializer">The <see cref="IRaygunMessageInitializer"/> to register.</param>
+    public virtual void AddMessageInitializer(IRaygunMessageInitializer raygunMessageInitializer)
+    {
+      if (RaygunMessageInitializers == null)
+      {
+        RaygunMessageInitializers = new List<IRaygunMessageInitializer>();
+      }
+      RaygunMessageInitializers.Add(raygunMessageInitializer);
+    }
+
+    /// <summary>
+    /// Removes a <see cref="IRaygunMessageInitializer"/> from the list of registered <see cref="IRaygunMessageInitializer"/> objects.
+    /// </summary>
+    /// <param name="raygunMessageInitializer">The <see cref="IRaygunMessageInitializer"/> to remove.</param>
+    public virtual bool RemoveMessageInitializer(IRaygunMessageInitializer raygunMessageInitializer)
+    {
+      return RaygunMessageInitializers.Remove(raygunMessageInitializer);
+    }
+
     /// <summary>
     /// Transmits an exception to Raygun.io synchronously.
     /// </summary>