diff --git a/src/Postmark.Tests/ClientSendingTests.cs b/src/Postmark.Tests/ClientSendingTests.cs
index 05cc7ae..cda08da 100644
--- a/src/Postmark.Tests/ClientSendingTests.cs
+++ b/src/Postmark.Tests/ClientSendingTests.cs
@@ -6,6 +6,7 @@
 using System.Linq;
 using System.Text;
 using System.Threading.Tasks;
+using PostmarkDotNet.Exceptions;
 
 namespace Postmark.Tests
 {
@@ -89,6 +90,25 @@ public async void Client_CanSendAPostmarkMessageWithEmptyTrackLinks()
             Assert.NotEqual(Guid.Empty, result.MessageID);
         }
 
+        [Fact]
+        public async void UnknownMessageStream_ThrowsException()
+        {
+            var inboundAddress = (await _client.GetServerAsync()).InboundAddress;
+            var message = ConstructMessage(inboundAddress, 0, null);
+            message.MessageStream = "outbound";
+
+            // Testing the default transactional stream
+            var result = await _client.SendMessageAsync(message);
+            Assert.Equal(PostmarkStatus.Success, result.Status);
+            Assert.Equal(0, result.ErrorCode);
+            Assert.NotEqual(Guid.Empty, result.MessageID);
+
+            // Testing an invalid non-existing stream
+            message.MessageStream = "unknown-stream";
+
+            await Assert.ThrowsAsync<PostmarkValidationException>(() => _client.SendMessageAsync(message));
+        }
+
         private PostmarkMessage ConstructMessage(string inboundAddress, int number = 0, LinkTrackingOptions? trackLinks = LinkTrackingOptions.HtmlAndText)
         {
             var message = new PostmarkMessage()
diff --git a/src/Postmark/Model/PostmarkMessage.cs b/src/Postmark/Model/PostmarkMessage.cs
index 1033963..0405a22 100644
--- a/src/Postmark/Model/PostmarkMessage.cs
+++ b/src/Postmark/Model/PostmarkMessage.cs
@@ -22,9 +22,9 @@ public PostmarkMessage()
         /// <param name="htmlBody">The HTML Body to be used for the message, this may be null if TextBody is set.</param>
         /// <param name = "headers">A collection of additional mail headers to send with the message. (optional)</param>
         /// <param name = "metadata">A dictionary of metadata to send with the message. (optional)</param>
+        /// <param name="messageStream">The message stream used to send this message. If not provided, the default transactional stream "outbound" will be used. (optional)</param>
         public PostmarkMessage(string from, string to, string subject, string textBody, string htmlBody,
-            HeaderCollection headers = null, 
-            IDictionary<string, string> metadata = null): base()
+            HeaderCollection headers = null, IDictionary<string, string> metadata = null, string messageStream = null) : base()
         {
             From = from;
             To = to;
@@ -33,6 +33,7 @@ public PostmarkMessage(string from, string to, string subject, string textBody,
             HtmlBody = htmlBody;
             Headers = headers ?? new HeaderCollection();
             Metadata = metadata;
+            MessageStream = messageStream;
         }
 
         /// <summary>
diff --git a/src/Postmark/Model/PostmarkMessageBase.cs b/src/Postmark/Model/PostmarkMessageBase.cs
index f408544..c0fd8cf 100644
--- a/src/Postmark/Model/PostmarkMessageBase.cs
+++ b/src/Postmark/Model/PostmarkMessageBase.cs
@@ -19,6 +19,11 @@ public PostmarkMessageBase()
             TrackLinks = LinkTrackingOptions.None;
         }
 
+        /// <summary>
+        ///   The message stream used to send this message.
+        /// </summary>
+        public string MessageStream { get; set; }
+
         /// <summary>
         ///   The sender's email address.
         /// </summary>
diff --git a/src/Postmark/PostmarkClient.cs b/src/Postmark/PostmarkClient.cs
index 449f354..0d02c5e 100644
--- a/src/Postmark/PostmarkClient.cs
+++ b/src/Postmark/PostmarkClient.cs
@@ -863,10 +863,11 @@ public async Task<PostmarkResponse> SendEmailWithTemplateAsync<T>(string templat
             bool? trackOpens = null,
             IDictionary<string, string> headers = null,
             IDictionary<string, string> metadata = null,
+            string messageStream = null,
             params PostmarkMessageAttachment[] attachments)
         {
             return await InternalSendEmailWithTemplateAsync(templateAlias, templateModel, to, from, inlineCss, cc,
-            bcc, replyTo, trackOpens, headers, metadata, attachments);
+            bcc, replyTo, trackOpens, headers, metadata, messageStream, attachments);
         }
 
         public async Task<PostmarkResponse> SendEmailWithTemplateAsync<T>(long templateId, T templateModel,
@@ -876,10 +877,11 @@ public async Task<PostmarkResponse> SendEmailWithTemplateAsync<T>(long templateI
             bool? trackOpens = null,
             IDictionary<string, string> headers = null,
             IDictionary<string, string> metadata = null,
+            string messageStream = null,
             params PostmarkMessageAttachment[] attachments)
         {
             return await InternalSendEmailWithTemplateAsync(templateId, templateModel, to, from, inlineCss, cc,
-            bcc, replyTo, trackOpens, headers, metadata, attachments);
+            bcc, replyTo, trackOpens, headers, metadata, messageStream, attachments);
         }
 
         private async Task<PostmarkResponse> InternalSendEmailWithTemplateAsync<T>(object templateReference, T templateModel,
@@ -889,6 +891,7 @@ private async Task<PostmarkResponse> InternalSendEmailWithTemplateAsync<T>(objec
             bool? trackOpens = null,
             IDictionary<string, string> headers = null,
             IDictionary<string, string> metadata = null,
+            string messageStream = null,
             params PostmarkMessageAttachment[] attachments)
         {
 
@@ -902,6 +905,7 @@ private async Task<PostmarkResponse> InternalSendEmailWithTemplateAsync<T>(objec
                 email.TemplateAlias = (string)templateReference;
             }
             email.TemplateModel = templateModel;
+            email.MessageStream = messageStream;
             email.To = to;
             email.From = from;
             if (inlineCss.HasValue)
diff --git a/src/Postmark/PostmarkClientExtensions.cs b/src/Postmark/PostmarkClientExtensions.cs
index 435fe0a..09775cb 100644
--- a/src/Postmark/PostmarkClientExtensions.cs
+++ b/src/Postmark/PostmarkClientExtensions.cs
@@ -10,7 +10,6 @@ namespace PostmarkDotNet
     /// </summary>
     public static class PostmarkClientExtensions
     {
-
         /// <summary>
         /// Sends a message through the Postmark API.
         /// All email addresses must be valid, and the sender must be
@@ -25,14 +24,16 @@ public static class PostmarkClientExtensions
         /// <param name="textBody">The Plain Text Body to be used for the message, this may be null if HtmlBody is set.</param>
         /// <param name="htmlBody">The HTML Body to be used for the message, this may be null if TextBody is set.</param>
         /// <param name="headers">A collection of additional mail headers to send with the message.</param>
+        /// <param name="messageStream">The message stream used to send this message.</param>
         /// <returns>A <see cref = "PostmarkResponse" /> with details about the transaction.</returns>
         public static async Task<PostmarkResponse> SendMessageAsync(this PostmarkClient client,
             string from, string to, string subject, string textBody, string htmlBody,
-             IDictionary<string, string> headers = null, 
-             IDictionary<string, string> metadata = null)
+             IDictionary<string, string> headers = null,
+             IDictionary<string, string> metadata = null,
+             string messageStream = null)
         {
-            var message = new PostmarkMessage(from, to, subject, textBody, htmlBody, 
-            new HeaderCollection(headers), metadata);
+            var message = new PostmarkMessage(from, to, subject, textBody, htmlBody,
+            new HeaderCollection(headers), metadata, messageStream);
             return await client.SendMessageAsync(message);
         }